When it comes to virtualization, you basically have a set of options on how you want virtualize your computers. You can choose to run your virtualized machines through libvirt, which is a a common interface to handle QEMU, KVM and XEN based virtualization methods through the command virsh. In addition when using libvirt you can install virt-manager which gives you a graphical user interface to manage and install virtual machines.
QEMU based virtualization is pure software emulation, where each guest machine has software based virtual devices and kernel. QEMU generates a more overhead than both KVM and XEN.
KVM utilizes virtualization technology present in the central processing unit. So you will need a newer cpu in order to fully utilize KVM. KVM also uses virtual devices and kernel, but does this in a much more effective way.
XEN on the other hand, don't virtualize the kernel or devices. Instead it runs directly on the host kernel. More on the architecture of XEN can be found here: Xen Architecture_Q1 2008.pdf
XEN and KVM is by far the most effective ways to virtualize computers. The main differences between them is that XEN works on all machines and hardware, while KVM needs virtualization technology present in the cpu. Also afaik, XEN is the only virtualization technology that supports pci/vga passthrough. Enabling you to run for example windows as a virtualized guest with full 3d acceleration as if windows was installed directly on the hardware. (videos and material on this here)
XEN is the technology used in Amazon's EC2 Cloud, but KVM seems to be the mainstream choice of several Linux distro's, like Ubuntu and RedHat.
I have personally tried all of these technologies, and my personal experience was that QEMU was too slow, XEN was a bit flakey for me, getting segmentation faults in several applications running after a period of time. But even though i experienced this problem, doesn't mean that you will. KVM on the other hands seems to perform well.
In my setup im running ubuntu 9.10 server with libvirt and kvm, the recommended setup of ubuntu. I have installed 5 virtual machines all with the ubuntu 9.10 server installation. The 5 virtual machines are as explained in part 1, netldap, netmisc, netvpn, netfw and netfiler. In this writing moment, netvpn and netfw are just clean installs and not put too use yet, and as for netfw, i will probably have to reinstall with another distro as smoothwall, and ipcop don't come as an deb package. In addition all virtual machines is running in bridged networking mode, giving each machines their own "external" ip address. For the more technical stuff on how i implemented virtualization, i suggest you take a look at https://help.ubuntu.com/9.10/serverguide/C/libvirt.html as there is no reason why i should repeat what is written the the ubuntu server documentation in this blogentry.