Both OpenVZ (first choice) and KVM (iSCSI+LVM).
I guess both (even though I have not yet managed to install Promox on my goddamn server F1DH)
But, the reason why people prefer Proxmox to OpenVZ/KVM is because it contains both.
I would love to see more features added to Proxmox though.. Like a little more work on Cloud (as what OnApp does)
It would then certainly hook the market.
Only KVM. OpenVZ is a buggy shit.