[Mr.Monkey]

As many others have pointed out already, the question can not really be answered. It depends on what you want to do.
If you just need process sepperation you should go with a container system like OpenVZ or any other, it is lighter you can (para)virtualize much more on the same hardware. It does not run its own kernel, it is basically just something like a jail for processes.
KVM on the other hand is different, it more or less simulates your own PC or Server inside your Sever, it runs it's on kernel which gives you the opportunity to install a totally different Operating System at the cost of resources.
It is like comparing a bike with a truck, more or less.
So it depends on your usecase, just want to run a webserver sepperated from your other applications, go with a container setup. Need to run another Operating System you have to go with full virtualisation.

Also older processors do not support full virtualisation at the cpu level, so your virtual machines will be slow, the only sensible thing there is to do a container setup.

No, OpenVZ is just an advanced chroot
It's not any lighter, it's just that resources are not dedicated so you can oversell. 
KVM doesn't technically run it's own kernel, it's just full virtualization not partial virtualization.

By the way, have you ever tried an OpenVZ container in a KVM VPS? It works

VM Inception ™