I was recently asked to comment on an article comparing xen vs kvm the question was if I think the two hypverisors can coexist or if only one will prevail long term.
KVM isn't really a hyper-visor in the traditional sense. Because it's based on Qemu it's more of a system emulator and utilizes a kernel patch/module to access the latest hardware virtualization to increase/improve performance. Long term, I think the better choice is the system that is easiest to install and use. Right now its KVM, I think Xen and KVM solve different problems, so there is room for both to flourish. Xen has more features, a larger community and better funding (Citrix). I would also note KVM isn't officially supported by Red Hat or Novell Suse although KVM has become the defacto standard for Ubuntu.
Given Xen's complexities, I'm recommending KVM to our more novice users, and Xen for the experts.