Xen vs. KVM, the hy­per­visors compared

In the age of cloud computing, hy­per­visors like KVM and Xen have become more crucial than ever. But how do these two vir­tu­al­isa­tion tech­no­lo­gies differ? With our Xen vs. KVM com­par­is­on, we’ll take a look at the sim­il­ar­it­ies and dif­fer­ences between the two.

Xen at a glance

Xen is open-source vir­tu­al­isa­tion software that makes it possible to sim­ul­tan­eously run multiple guest operating systems on a single host machine. Xen manages the al­loc­a­tion of the hardware resources on the host machine.

What type of hy­per­visor is Xen?

Xen is a Type 1 (or bare metal) hy­per­visor. This type of hy­per­visor is installed directly onto a physical system and com­mu­nic­ates with hardware through its own drivers. Host and guest operating systems are installed above the Xen hy­per­visor. To dif­fer­en­ti­ate between different levels of privilege, Xen cat­egor­izes the operating systems that are installed into two different domains: dom0 and domU. The host operating system belongs to the dom0 domain and enjoys the highest level of privilege, while all guest operating systems are clas­si­fied as un­priv­ileged systems and belong to the domU domain.

The host operating system doesn’t com­mu­nic­ate directly with the hardware through its drivers but instead through the hy­per­visor using a spe­cial­ized Xen API. This deep in­teg­ra­tion ensures more efficient use of the hardware, less overhead and a reduced attack surface.

How Xen uses para­vir­tu­al­isa­tion to boost ef­fi­ciency

Xen uses the vir­tu­al­isa­tion technique para­vir­tu­al­isa­tion to boost ef­fi­ciency. With this technique, the hy­per­visor doesn’t emulate hardware com­pon­ents, and guest systems are aware that they are operating in a vir­tu­al­ised en­vir­on­ment. This allows for much faster com­mu­nic­a­tion between guest operating systems and Xen, leading to increased per­form­ance.

However, this approach relies on spe­cial­ized drivers and specific built-in cap­ab­il­it­ies in the operating system kernel. Many widely used operating system kernels such as Linux and BSD have been able to support para­vir­tu­al­isa­tion for years.

If you are using an operating system where para­vir­tu­al­isa­tion is not possible or not desired, Xen also supports full vir­tu­al­isa­tion (where hardware com­pon­ents are virtually emulated). Although this slows down vir­tu­al­isa­tion, it provides high com­pat­ib­il­ity, allowing operating systems that do not support para­vir­tu­al­isa­tion to run under Xen.

What are KVM’s key features?

Like Xen, KVM (Kernel-based Virtual Machine) is also an open-source hy­per­visor that lets users run multiple guest operating systems on a single physical machine. While both hy­per­visors serve the same fun­da­ment­al purpose, their ar­chi­tec­tur­al design is different, with each hy­per­visor being suitable for different use cases.

What type of hy­per­visor is KVM?

In the past, KVM was often referred to as a Type 2 hy­per­visor, because it could only com­mu­nic­ate with hardware through the drivers of the host operating system. KVM had to emulate all hardware com­pon­ents for the guest systems, making para­vir­tu­al­isa­tion im­possible.

KVM has since been expanded to include para­vir­tu­al­isa­tion. The way that it operates is different to Xen though. Unlike Xen, KVM is not installed under the host operating system but instead directly in­teg­rated into the Linux kernel. KVM uses the software com­pon­ents of the Linux system to manage vir­tu­al­ised systems and their processes.

Under KVM, each guest operating system has its own vir­tu­al­ised set of hardware, including network in­ter­faces, disk storage and other types of hardware com­pon­ents. To ac­cel­er­ate full vir­tu­al­isa­tion, KVM employs hardware-assisted vir­tu­al­isa­tion, which is already present in most modern pro­cessors.

Not every component undergoes full vir­tu­al­isa­tion in KVM. To enhance the per­form­ance of running systems and ap­plic­a­tions, KVM provides para­vir­tu­al­ised in­ter­faces that can directly com­mu­nic­ate with the hy­per­visor through the Virtio API. KVM pre­dom­in­antly uses para­vir­tu­al­isa­tion for input and output devices like the network interface. That’s why when dis­cuss­ing KVM, people often describe it as employing partial para­vir­tu­al­isa­tion.

KVM vs. Xen, how their per­form­ance compares

The main dif­fer­ence between the two hy­per­visors lies in where they operate within the software stack. KVM is in­teg­rated into the Linux kernel and uses hardware-assisted vir­tu­al­isa­tion to vir­tu­al­ise numerous hardware com­pon­ents without ex­per­i­en­cing sig­ni­fic­ant per­form­ance drawbacks. The primary IO in­ter­faces are para­vir­tu­al­ised, which, for most server ap­plic­a­tions, is probably the most crucial aspect.

Xen, on the other hand, is tra­di­tion­ally installed under the operating system as a Type 1 hy­per­visor. This means that the hy­per­visor manages the dis­tri­bu­tion of hardware resources itself. The­or­et­ic­ally, this allows for Xen to provide more efficient and per­form­ant com­mu­nic­a­tion with the hardware. However, KVM’s Virtio drivers and hardware-assisted vir­tu­al­isa­tion put its level of per­form­ance on par with Xen.

CPU pinning

Despite ex­hib­it­ing sim­il­ar­it­ies in per­form­ance levels, the two hy­per­visors do not perform equally in all scenarios. When it comes to CPU pinning, KVM offers more power to its users. With CPU pinning, KVM can assign a physical processor (or a processor core) to a vir­tu­al­ised processor, ensuring exclusive usage for that virtual system. Of the two hy­per­visors, only KVM provides this feature, which is par­tic­u­larly be­ne­fi­cial for CPU-intensive ap­plic­a­tions.

Network per­form­ance

In terms of network per­form­ance, Xen takes the lead. Even though both hy­per­visors provide para­vir­tu­al­ised network in­ter­faces, each system vir­tu­al­ised under KVM has its own network device. With Xen, all guest systems share a single virtual network interface, which leads to increased ef­fi­ciency.

Summary

The question of which hy­per­visor is better depends on your re­quire­ments. The market leader KVM is in­teg­rated into the Linux kernel and offers the pos­sib­il­ity of full vir­tu­al­isa­tion. These aspects make it fun­da­ment­ally easier to use.

However, Xen’s full para­vir­tu­al­isa­tion cap­ab­il­it­ies means that it can the­or­et­ic­ally provide better per­form­ance. The crucial factors you should consider when choosing a hy­per­visor are the ap­plic­a­tions you want to vir­tu­al­ise and the level of ex­per­i­ence you have.