XEN vs KVM

I’ve been running some benchmarks on KVM and XEN of late and here are some views on it. Remember we are comparing XEN pv vs KVM. But there is nothing wrong in it, as we should choose the best of both to compare them truly.

XEN

XEN is extremely active even today even after Redhat adopting KVM, even after Fedora community has not shipped a dom0 with fedora 9 and that they would altogether stop providing XEN from fedora 10 onwards, and KVM being chosen as the default in Ubuntu 8.04. 2.6.25 paravirt_ops enabled domU on XEN is by far the post that has received the largest number of hits!

My experience with XEN was not good in the beginning with my Network device not working, (solved here), AGP not detected, among others.  The officially distributed xen dom0 is on a 2.6.18 kernel which were used by dinosaurs, ages ago. Fortunately the support for XEN domU is in the mainline. Fedora used to do all the hardwork to rebase 2.6.18 domU to a newer kernel, but due to high maintenence costs, they have stopped it. The good things about XEN are,

• XEN performs extremely well with microbenchmarks giving upto 90% of the performance of native, which is excellent.
• XEN has excellent management tools -xm.
• XEN user and developer community is very active with almost 150 mails a day.
• Has a bigger corporate presence.

KVM

KVM found itself in the kernel in as short as 6 months after is launch. This is a BIG plus to KVM. Now KVM ships as an official kernel module which means less maintenence for the distro creators. It uses linux kernel as its hypervisor and does not duplicate scheduler and memory management code. But one drawback is that it does not work on CPUs that dont have hardware virtualization support (before core 2).

Now lets talk of some disadvantages.

• Not very stable yet. Real mode evaluation does not work perfectly on Intel machines and is being worked on a SoC 2008 project.
• Runs with QEMU. I’ve seen people with an opinion like “Customers are going to laugh at you if you ask them to run QEMU”. I dont see how far this is true. But it could still be a valid point.
• Performance is not very good.
• Came late into the scene.

Now, if you are trying to decide between the two, I’d recommend KVM. I believe KVM is going to become more prominent in the future because the paravirtualization techniques will be outperformed with hardware becoming smarter at virtualization. Also, RedHat and Canonical have begun supporting KVM. Also, KVM performance is improving day by day. Its not really late. There’s still a lot of juice left in the virtulization market!

Good luck KVM!