[CentOS] xen vs kvm for virtualization on centos/rhel?

> I recommend using VirtualBOX from Sun. Close to wire speed, no need to alter
> the kernel. Simple and flexible to use.

I use VirtualBox on customer sites in order to virtualize a CentOS
instance because it runs on Windows during the implementation phase,
and then we can easily sneak it on their Solaris servers when it goes
in production.
I would never have been able to introduce a Linux over there without
Virtual Box...

It works mostly fine and is the best FLOSS solution I have seen for
desktop virtualization, but I don't find it rock-solid and they tend
to release very often with significant changes. It feels like a Fedora
rather than a CentOS, if you see what I mean.
But updates have become much easier to follow since they set up a yum
repository.

As per the OP question, I started preparing a virtualized
infrastructure with CentOS 5 hosts (and guests) approximately one year
ago and went the KVM way for the reason already described (mostly that
it seems to be Red Hat long-term strategy).
I never had any issue with KVM/QEMU/libvirt (so far...) and it is very
easy to automate with virsh and the XML configurations (just as Xen I
guess).