What is a Hypervisor? The Basics of Virtualization Technology

What is a hypervisor, what is virtualization and what are they used for? In this article, we will explain the basics of virtualization, describe the types of this technology and share some details on real-world application from SIM-Networks’ experience.

What is a hypervisor and what is it used for?

A hypervisor is a program, which controls the physical resources of a computer or a server and distributes those resources between a number of various operating systems, which allows them to run at the same time. The hypervisor is the software that enables this process, while the process itself is called virtualization.

Virtualization is the distribution of server resources between multiple virtual segments. The physical machine is called the host; it has a specific CPU, RAM and storage configuration. These resources are distributed in such a way that each virtual segment can run like a separate computer with its own operating system, which is called the guest OS.

For example, the SINGLE server from SIM-Networks’ line of dedicated servers has 4 CPU cores, 16 Gb of RAM and two 500 Gb drives. You can imagine it as two virtual servers with 2 CPU cores, 8 Gb of RAM and one 500 Gb drive each.

Let’s take a server for a small e-commerce project: the company needs to host its site, ERP system and a couple of remote desktops for employees. If the server is powerful enough, you can divide the resources between these tools using a hypervisor. This way, the site, the ERP and the remote desktops will have their own dedicated resources running their guest operating system. It will appear as though each process runs on a separate server, even though the company is using only one physical machine.

A schematic representation of a virtual machine

How do hypervisors work?

A hypervisor creates copies or clones of one physical computer’s hardware resources. Each clone is presented to the user as a separate device. Users can install guest operating systems on each of the virtual machines with no ties to the host hardware.

A hypervisor isolates the operating systems from each other so that each one gets exclusive use of the resources dedicated to it. If needed, a hypervisor can allow the operating systems of separate virtual machines to interact. This connection can be used for shared access to certain files or exchanging data through the local network. This way, one computer appears to be multiple computers at the same time, and each works with its own software independent of the others.

There are two main types of hypervisors; however, there are also hybrid hypervisors, which combine the features of both types.

How do Type 1 hypervisors work?

The principle of operation of a Type 1 hypervisor

Type 1 hypervisors are also called microkernels, thin hypervisors or autonomous hypervisors run on bare metal. Type 1 hypervisors are best imagined as a compact operating system with specific functions, which is installed directly on a bare-metal server and has the main signs of an OS:

• presents an abstract set of resources for applications instead of an unordered set of hardware;
• controls the resource pool by distributing CPU time, memory, I/O devices between programs which request the use of the computer’s resources.

Security is one of the main requirements towards hypervisors, as they get full control over the hardware resources on which virtualization is run. Consequently, the hypervisor’s task is to run the machine code in a safe way while not allowing a guest OS to execute commands directly on the host machine or modify the resources reserved for other virtual machines.

Xen, VMware ESXi, Hyper-V, KVM and other hypervisors can be considered Type 1.

Xen (Xenserver, Citrix Hypervisor)

The Xen thin hypervisor was developed as part of a research project of the Cambridge University laboratory. XenSource was absorbed by Citrix in 2007; as a result, some of the products were renamed. Xen is a crossplatform hypersor which supports hardware virtualization and paravirtualization. Most of the components are located beyond the hypervisor, which is why Xen contains a minimal amount of code. Xen is a fully open-source GNU GPL 2-licensed hypervisor. This means that the product can be modified however the user needs. Some consider Xen a hybrid hypervisor due to its support for paravirtualization and hardware virtualization.

VMware ESXi

The autonomous VMware ESXi hypervisor is an Enteprise-class virtualization solution developed by VMware. Earlier, a limited version of ESXi was available for free; however, since VMware was acquired by Broadcom, the new owners dropped support for this version. The hypervisor is now only available for free within a trial period. The licensed version offers a broad set of features, such as centralized VM management on all host machines within a project using the vCenter platform. The free version, however, still performed the basic functions of a hypervisor and was one of the most popular virtualization solutions on the market.

Hyper-V

Hyper-V is hardware virtualization solution for x64 systems made by Microsoft. It is available in two variants: as a role in the server operating systems of the Windows family and as a separate product – Microsoft Hyper-V Server. When installing Hyper-V on OS Windows, the user might get the impression that the hypervisor is being installed directly on the operating system which is already running on the “bare metal”, as is the case with Type 2 hypervisors. In fact, this hypervisor is installed a level below, and the installed operating system becomes a guest OS running on one of the virtual machines. Hyper-V is one of the most popular hypervisors among SIM-Networks clients who are used to working with Microsoft products: this hypervisor was mainly made to work with Windows guest OS’.

KVM

Kernel-based Virtual Machine, KVM is a hypervisor created in October of 2006. It was quickly integrated with the main branch of the Linux 2.6.20 core. Later, KVM was adapted as a core module in FreeBSD. KVM is fully open-source through the GNU GPL and GNU LGPL license, which is why KVM QEMU was used to create SIM-Cloud. Using open-source software allows us to modify the hypervisor as needed and stay independent from vendors. KVM is also popular among SIM-Networks clients who order unique IT infrastructures built from scratch.

What are the main principles of Type 2 hypervisors?

Type 2 hypervisors are also called hosted hypervisors. This type of hypervisor is an additional software layer, which is located above the main OS.

These hypervisors run as one of the processes executed by the main OS, most commonly Linux. In this case the hypervisor’s rights are significantly limited. It controls the guest operating systems, while emulation and physical resource control are handled by the host OS.

The principle of operation of a Type 2 hypervisor

Oracle VM VirtualBox, VMware Workstation are the most popular Type 2 hypervisors.

Oracle VM VirtualBox

Oracle VM VirtualBox is a modular crossplatform hypervisor for Linux, macOS, Microsoft Windows, FreeBSD, Solaris/OpenSolaris, ReactOS, DOS and other Oracle systems. The hypervisors was created in 2007 by Sun Microsystem; after Oracle absorbed the company, work on the hypervisor continued. The source code of the basic version was opened through the GNU GPL license; the hypervisor is popular due to the unlimited modification capabilities. However, SIM-Networks administrators note that this hypervisor is rarely used for virtualization in corporate settings: its set of features is better-suited towards software testing and practice.

VMware Workstation

The first version of the VMware Workstation hypervisor, developed by VMware, was released in 1999. This proprietary software works x86 and x64 host operating systems, such as Microsoft Windows, Linux, Ubuntu, CentOS. It supports more than 200 guest operating systems. Earlier, Workstation Player was available as a free trial version; however, in early 2024 VMware made the Pro version free for personal use. Any commercial use of the hypervisor still requires a license.

What are hybrid hypervisors?

Hybrid hypervisors combine some of the characteristics of Type 1 and Type 2 hypervisors (a combination of a thin hypervisor and the specialized service OS functioning on the hardware level under its control). The hypervisor controls the CPU and the memory directly while the service OS provides guests access to I/O devices.

This technology is in active development; more flexible and integrated versions are created constantly. Lately, Xen and Hyper-V have been classified as hybrid hypervisors instead of Type 1, which partly correct. The modern versions of these hypervisors combine a significant number of characteristics from both types.

What is paravirtualization?

Paravirtualization is a popular solution which installs a pre-configured guest OS, the core of which is modified for effective work with Type 2 hypervisors. Naturally, this does not concern closed source proprietary systems, such as Windows. However, most Linux versions can be modified freely.

Guest operating systems are modified to run in a virtualized environment; the main condition for paravirtualization is the open source code of all OS components. However, the significant increase in performance, which is comparable to that of a physical system, drives the demand for paravirtualization.

What are container solutions?

In the past couple of years, container technology has started to compete with hypervisors in the field of virtualization. The reason behind this is that containers can deploy a larger number of applications on one physical server compared to a hypervisor. Container virtualization solutions are mostly based on the modified Linux core. If the host machine uses a Linux core, guest operating systems can only come from the Linux family.

OpenVZ is popular container solution, which was used to create the Virtuozzo platform. The main benefit of the OpenVZ solution is increased performance and maximal physical resource usage due to the high density of VM placement. The Jailhouse container solution by Siemens is also interesting. This hypervisor functions on bare metal; however, it is launched on a running Linux system. The solution segments the system into isolated cells, parts of the system necessary for running user applications.

What hypervisor should I choose?

Qualified experts can help pick the best virtualization technology for each user case. They will take your requirements towards the project into account, as well as the existing hardware, the price, the availability of technical support and other important factors. For example, when planning to use VMware ESXi and some other hypervisors, be ready to purchase licenses and to buy high-quality physical equipment for the virtualization system.

Before starting to work with a hypervisor, a user should:

• dedicate some time to learn all relevant information on the software developer’s website;
• check if your hardware platform meets the minimal technical requirements of the hypervisor;
• analyze other properties of the chosen software solution so that expectations match reality.

If you have any questions regarding the choice of an optimal virtualization technology, contact SIM-Networks; we will gladly consult you and answer your questions.

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