Xen is great. In a world without hardware virtualization you could run virtual machines and have multiple workloads isolated on commodity hardware. What a huge step forward!
A few years later, a lot of people have done that. But an ever greater solution evolved: KVM. Today, you can just run VMs as processes on Linux. No more fiddling with special hypervisors or learning new commands. Everything just integrates.
With that situation, we've run into a tough place though. People have virtual machines configured and running on Xen. They use special PV kernels that don't work in a KVM virtual machine and access PV hardware that KVM doesn't expose. Or does it?
As of Qemu 0.15 you can run your Xen PV virtual machine on KVM. Just like that. With all the ease of use, Linuxiness and no guest modifications at all.
This talk tells you how it works and how you can use it.