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Xen vs. KVM, the pros and cons, from LinuxWorld 2007

Software engineers Jeremy Fitzhardinge of XenSource Inc. and Avi Kivity of Qumranet Technologies laid out the pros and cons of Xen and KVM in a LinuxWorld 2007 session, titled “Xen and KVM: Separating fact from fiction” just a few minutes ago. Panelists also included moderator Sunil Saxena and Jun Nakajima of Intel Corp., but I’m focusing on Fitzhardinge’s and Kivity’s takes on the facts about both virtualization technologies.

Let’s start with KVM.

“KVM is based on Linux which is the most scalable operating system on the planet. It shows very good scalability in terms of guests. We expect to be able to run very large guests with great scalability soon,” Kivity said.

KVM tries to leverage hardware virtualization and Linux kernel capabilities to the maximum, Kivity said. KVM leverages Linux memory management and I/O capabilities and its scheduler and security model. Real-time scheduling is made possible with KVM. It’s also easy to use existing management tools with KVM.

“There is work being done to get hardware assistance in KVM,” Avity said. In the meantime, KVM requires hardware assistance. Other cons include a lack of flexibility in device assignment and needs work in memory and power management.

Xen, on the other hand, can run only any platform without hardware support. However, without hardware support, performance can suffer, said Fitzhardinge. In most situations, hardware support is available, so this isn’t a big deal. Xen does need to draw from Linux support for power management and platform initialization.

“Xen has minimal support for power management at this point,” Fitzhardinge said.

On the plus side, Xen has great security isolation, is operating system independent and supports multiple hardware platforms. Xen is designed around paravirtualization, which gives it low overhead and good performance, Fitzhardinge said. Also, Xen supports both paravirtualization and hardware-assisted virtualization.

Unfortunately,KVM lacks manageability features that exist in Xen today, platform initialization with Xen is complex and performance on 64-bit processors isn’t great, Fitzhardinge said.

So, overall, scalability in number for guests and VCPUs is good on both Xen and KVM; but KVM supports power management and Xen does not. KVM has broad machine support, but Xen does not. Both have Linux vendor and community support.

Looking ahead, Xen and KVM developers plan to add enhanced backward compatibility and share I/O device support. Interoperability is also a concern that both development groups will work on. On the full virtualization side, machines images installed on one hypervisor will be obtainable on the other, Kivity said. To what extent the hypervisor will present the same hypervisor interface is the question. At the moment the Xen interface is quite different from the KVM interface, said Fitzhardinge. As Xen goes towards more hardware support, there are more possibilities for developing interoperable interfaces.