Hardware: Intel® Core™ i7-920, 12 GB DDR3 RAM, 2 x 1500 GB SATA-II HDD (no SoftRaid because Proxmox developers don't support softraid and they are sure you'll run into problems)

Software: Proxmox VE with KVM and OpenVZ support and debian everywhere

I want to run multiple Linux VMs on this server. One for a firewall (I want to try pfSense), one for MySQL, one VM for nginx (my stuff) and ~2 VMs with nginx for other people's web sites.

I don't think that pfSense will run in an OpenVZ environment but it should run in KVM.

The question is if I should setup the other VMs using KVM or OpenVZ. In OpenVZ they should have less overhead for the OS itself but I don't know about the performance. I heard that KVM is more stable but needs more RAM and CPU.

I found this diagram showing a OpenVZ setup on the same hardware I'm using. This guy uses an own VM for each and every website which is running on his server. I can't think of any advantage why he's using so many VMs.