I think, its just the matter of preference but it also depends on what type of applications you want to host on VPS, how much will be utilization etc.
I've worked on all three (OpenVZ, XEN and KVM) technologies and they have their own pros and cons.
But, as you've specifically asked about XEN or KVM then I would recommend you to go with KVM due to its architecture, provide isolation and resources are dedicated to it.
Its bit complex but with appropriate optimization of VPS and installation drivers, KVM would work great.
I hope it helps.