Synthetic benchmarks considerations
The synthetic benchmarks help us to examine the various subsystems one by one.
We are going to examine the subsystems in the following order:
cache and memory speed
I/O disk access speed
While synthetic tests are invaluable in an in-depth performance analysis, remember that they describe only half of the truth and, sometime, can even be misleading: for example, a software that run a CPU benchmark very fast can be quite slow in real-world situations, where many factor are at play at the very same time.
UPDATE: a recent article comparing KVM vs VirtualBox can be found here: http://www.ilsistemista.net/index.php/virtualization/12-kvm-vs-virtualbox-40-on-rhel-6.html