Xen is much faster in my experience because everything has been paravirtualized since the beginning. KVM devs realized that Intel's hardware virt wasn't all it was cracked up to be and now it too runs in hybrid mode when possible.
Xen is proven, stable, and it works on a lot of hardware that KVM doesn't. It is used by companies like Linode.com and Slicehost with great success. Having the latest Dom0 kernel would be a boon to usage and latest kernel features/hw support.
KVM is great, especially for desktops, but ATM Xen is widely used on servers and shows no sign of reduction. Please merge!