With linux kernel 2.6.20 and upwards providing full virtualization solution via the Kernel Virtual Machine (KVM), it is best to learn what you can and cannot do with this new feature.
M. Tim Jones provides an in-depth study of the KVM architecture and its advantages compared to other virtualization solutions.
Whilst I have not fully explored this yet, I pretty happy with the way Parallels Workstation for Linux handles virtualization on userland. However, I am pretty sure that if and when Parallels decide to take advantage of KVM, it will have a significant performance boost considering KVM's tight integration with the kernel.
So what are you waiting for? Start throttling those CPU cores to maximum! :)