I am using KVM for a while now and reinstalled my XEN-Server. I have to mention it's not the same hardware. XEN was on a 2 year old Server with 4G memory and Debian Lenny32, KVM is on a 6 months old 8G memory Server with Debian Squeeze. So i have a bit better hardware and a 64bit System on KVM.
I like that KVM is out-of-the-box working with virsh/virt-manager the performance improvement is enormous, especially with 64MB-RAM systems like I use for my wiki. I use LVM for the guest-disks (I did it even for the XEN-guests) and compared to use files it's even a huge performance improvement.
The only thing that's easier in XEN is that you can mount the guest-partitions and can access them directly in KVM you need to use kpartx (not hard but one step more) to achieve this.
I started to move to KVM because it seems as XEN development slowed down and there is much more development activity and support for KVM. I think that was a good decision.