I've been using OpenVZ quite happily to run multiple instances of linux, but now I'm checking out Xen and KVM as other virtualization options. I just read up on both, and at this point I'm going with Xen, mainly as my target system (the Intel Little Valley "1") doesn't support VT.
My goal here is to run debian as the base DomU, and then run additional linux instances on top of it, as well as FreeBSD. I'm curious as to how there can be multiple kernels on a single machine, and I'm wondering if Xen is at all like QEMU. I've also had a lot of success with QEMU, but I found that networking with VDE2 to be too confusing.
Speaking of which, I should probably look into Xen networking before I spend too much time on it. I just found this page on Xen Networking and it appears to be similar to OpenVZ, which I like, at least when you setup the virtual environments to use a virtual ethernet device.