FWIW: The answer depends entirely on your needs, now and in the future.
Yes, I know that's an unhelpful answer. Unfortunately, it's true. Your choice of virtualisation will affect pretty much everything you do afterwards, so you need to ask yourself a few questions.
(1) Is the difference between 97% of native performance and 96% of native performance (figures plucked out of the air) really that important to you?
(1a) If one does better with HD access (which really implies that you're either using a whacking great database, or you can't afford the extra RAM), and the other does better with networking, which is more important to you?
(2) Are you confident using the tools provided with either solution?
(3) Does the fact that one is (sort of) native to recent Linux kernels, and the other one not, make a difference?
(3a) Do you now, or have you ever been ... er ... will you ever need to run a different OS under virtualisation? It doesn't have to be Windows. It might be FreeBSD, or even Haiku, or whatever. (Xen probably wins here, but I suggest you check.)
Looking at the big picture, I see KVM as Linux' answer to Solaris zones. (I'd rather have Solaris zones, but I see the parallel.) I see Xen as a mature hypervisor technology with support for multiple OSes, but then if you don't need multiple OSes, that doesn't matter so much.
To be absolutely honest, you can't go far wrong either way (given caveats above). I prefer Xen, because I went to Cambridge; but then, if I worked for RH, I'd probably prefer KVM.