I just came across a very simple virtualization benchmark. This kind of benchmark shows some of the problems with virtualization: it usually works pretty well at limited load, but you pay the price at high load. This appears to be true of any virtualization technology today, even if it varies in the details.
Benchmarking often boils down to a petty fight. One get a true sense of just how reliable benchmarks can be by comparing the Xen and the VMware results for what appears to be the same benchmarks (but, oddly enough, with opposite conclusions).
Unfortunately, hardware assistance for virtualization, like Intel VT and VT-i technologies, does not seem to help much there (yet). It seems like it was designed primarily to make virtualization easier. As a result, we get for example Xen running Windows guests. But it does not really help with performance (summary).