Xen vs KVM: hypervisors under study

The Xen hypervisor in brief

The Xen hypervisor is open source virtualization software that allows you to run multiple guest operating systems simultaneously on a single host computer. The allocation of host computer hardware resources is taken care of by the Xen hypervisor.

What type of hypervisor is Xen?

As a hypervisor type 1 or bare metal, Xen is installed directly on the physical system. It communicates with the hardware through its own drivers. The host and guest operating systems are then installed on top of the Xen hypervisor. Xen assigns installed operating systems to domains dom0 And domU in order to distinguish between the different privilege levels. The host operating system is owned by dom0 and therefore has the highest privilege level. All guest operating systems are owned by domU as “underprivileged” systems. unprivileged).

Any communication between the host operating system and the hardware, however, does not take place directly via the operating system drivers, but via the hypervisor by means of a Xen API special. Deep Xen hypervisor integration enables more efficient use of hardware, reduced overhead, and reduced attack surface.

Xen: improving efficiency with paravirtualization

One virtualization technique that Xen uses to increase efficiency is paravirtualization. In this case, the hardware components are not emulated by the hypervisor : the guest system is therefore aware that it is running in a virtualized environment. On the other hand, communication between guest operating systems and the Xen hypervisor is much faster, which helps increase performance.

This technique, however, places special requirements on operating systems installed on Xen: among other things, support for paravirtualization in the operating system kernel as well as specialized drivers are required. These two requirements have already been met for years by common operating system kernels like Linux and BSD.

If paravirtualization is impossible or undesired for an operating system, Xen also supports complete virtualization, in which hardware components are emulated virtually. This slows down virtualization, but in return offers higher compatibility. Thus, even operating systems that do not support paravirtualization can be run under Xen.

The KVM hypervisor in brief

KVM (from English Kernel-based Virtual Machine) is also an open source hypervisor with which one can run multiple guest operating systems on a single physical computer. Although KVM and Xen fundamentally serve the same purpose, they are built according to different operating principles and therefore also suit different use cases.

What type of hypervisor is KVM?

In the past, KVM was often described as a type 2 hypervisor, because it could only communicate with the hardware through the host operating system drivers. Paravirtualization was therefore impossible and all hardware components had to be emulated by KVM for the guest systems.

In the meantime, the paravirtualization option has been added to KVM, but it works differently from Xen. Unlike Xen, KVM is not installed under the host operating system. Instead, it is directly integrated into the Linux kernel and thus uses the software components of the Linux system to manage virtualized systems and their processes. Each guest operating system has its own virtualized hardware under KVM, including network interface, disk space, etc. To accelerate this complete virtualizationKVM uses what is called hardware-assisted virtualization (in English hardware-assisted virtualization), which is already integrated into most modern processors.

But not all components are fully virtualized under KVM. In order to speed up running systems and applications, KVM offers some paravirtualized interfaces which can communicate directly with the hypervisor by means of theVirtio API. KVM paravirtualization is therefore mainly proposed for input and output devicesas for the interface network. With KVM, we are therefore rather talking about a partial paravirtualization.

KVM vs. Xen: Which Hypervisor Offers Better Performance?

The biggest difference between the two hypervisors is the level of the software stack in which they operate. As an integral part of the Linux kernel, KVM uses hardware-assisted virtualization to fully virtualize many hardware components without a significant performance penalty. The main input/output interfaces are thus paravirtualized, which should be the most important factor for most server applications. On the other hand, Xen, as a type 1 hypervisor, is typically installed below the operating system concerned. The hypervisor therefore manages the distribution of hardware resources itself. This do that Xen can theoretically offer communication more efficient and more efficient with the material. However, Xen and KVM have similar performance thanks to KVM's Virtio driver and hardware-assisted virtualization.

CPU pinning

Of course, the two hypervisors do not offer the same performance for all applications. It is especially in terms of CPU pinning that KVM offers its users more power than Xen. Using CPU pinning, KVM can assign a physical processor (or processor core) to a virtualized processor, so that only that virtual system can use the processor. This is particularly advantageous for applications loaded in CPU and is only possible with KVM.

Network performance

When it comes to network performance, Xen has a head start. Although both hypervisors provide paravirtualized network interfaces, each KVM virtualized system has its own network device. With Xen, all guest systems share a single virtual network interface, providing increased efficiency.

Ken vs KVM in summary

Which of Xen or KVM is the better hypervisor? The answer will depend on your needs. Thanks to its market leading position, its integration into the Linux kernel and the possibility of full virtualization, KVM is easier to use. Xen, on the other hand, has a higher theoretical performance thanks to its complete paravirtualization. To determine your choice, take into account the applications you want to virtualize on the one hand and your level of experience on the other hand.