You need to have Intel or AMD's virtualization support to take advantage of KVM.
Even with the virtualization support KVM will be slower then PV. Xen's PV is very superior in terms of performance in almost all situations.
KVM's advantages over Xen are:
* Cleaner design. I am guessing that KVM hypervisor code is between 20k-30k with all the arch it supports were Xen's hypervisor code is easily 10x that much.
* Much easier to administrate and deal with. Does not require patches, does not require rebooting or anything of that nature. It's "just there". Does not require special console software or management tools beyond just qemu if that is all you want. You can use top to monitor VMs and crtl-z to pause them if you started them from a terminal, for example.
* Does not require to have your OS "lifted" into a Dom0... The way Linux interacts with the hardware does not change. This means (with latest kernels) I can suspend my laptop while running VMs and it just works.
* Heavily leverage's Linux's existing features. Instead of having to write various peices of hardware support into the hypervisor KVM gets all that and more by default. When Linux does improvements to, say, memory management then people using KVM directly benefit from that work.
(this is not a huge advantage over Xen, its more of a big improvement when compared to Vmware ESX.. no restrictions to hardware, network block protocols, or sata or anything like that... if Linux supports it you can use it in KVM)
* It is already installed and setup on your machine. All you have to do is intall the qmeu portion and the virt-manager or libvirt stuff if you want to have a nice and easy way to manage them. All Linux distributions have KVM support.. it's modules are by default by everything I've looked at.