First of, no, I'm running it with 2 nics, one internal and one external (and a third DMZ which is purely virtual).
Hmm, Hypervisor is a general term for the software that creates the virtual environment, are you thinking about Hyper-V (from M$), then it's definetely not the fastest. They will perform windows virtualization better than other 'non-para-virtualized' hypervisors.
You can gain a lot of performance enhancements by doing it para-virtualized, which hyper-v wants to do with Windows (and Linux, but with problems), a much more compatible product is XEN, that will do para-virtualized for a lot more platforms, and do it better.
Only problem is that with paravirtualized you have some system drivers that can completely crash all virtual machines, and render them unsalvagable (speaking of experience), which was why we switched all virtual environments to VMWare a year ago at work.
But para-virtualized systems requires specially compiled kernels, and special drivers, so going with hyper-v you are really locking yourself down to M$ until all the kernels are available, which for us was an absolute nono.
