Qemu is a hardware emularor, Xen, VMWare, KVM are virtualizers.
If you want virtualization for learning purposes, I would say, test them all.
If you need industrial/commercial grade virtualization, this means reliable hardware and reliable software support, see what the big boys have to offer.
Server virtualization is something you would not run on your kid's super quad core play system.
I've seen server offers at Sun, at prices far below "PC Compatible" best offers.