FTP server test
For evaluate FTP performance, I installed FileZilla on each guests and then I uploaded to, and downloaded from, it the ISO image of the Ubuntu 9.10 x86_64 live CD.
Here are the results expressed in KB/s:
All in all, I think that the all virtualizers tested are in the same league here.
It can be that CPU usage can paint a different picture?
Please note that the bottom half show you CPU load during upload, while the top half during download. The results are not so homogeneous now: Xen is the least efficient hypervisor here, followed at some distance by KVM (remember that interrupt time is already included in privileged time). It's a shame, because its IRQ service time seems to be very low, but its total privileged shows that the syscall routines execute very slowly.
Speaking about KVM, we can see that it seems to be quite better that Xen, but it remain over 2X heavier than VMware and VirtualBox, which are the true leaders here. VMware's very low interrupt time is again a gift of its paravirtualized network driver, but VirtualBox is very good also: it is only in the download test that VMware is capable of pulling a bit ahead.
What about the hard disk load?
The first thing to note is that the download disk activity is so small thanks to the guest side caching (I run download tests after the upload ones and, while I purged the host sided cache, the guest cache was left intact).
Upload disk activity is a very different beast: we can see that VMware is the most efficient machine, Xen is the least, and VirtualBox and KVM are somewhere in the middle.
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