This is bound to spark of a huge discussion. Does that also makes hypervisors inefficient for CloudApps?
The hypervisors we tested from VMware (ESX 3.X), XenSource (Xen) and Microsoft's Virtual Server (not Hyper-V, which wasn't yet released) all prohibit the ability for the CPU/chip sets to reduce speed or go into "green mode" as long as there's a virtual machine guest running atop the hypervisor -- regardless of operating system flavor.
Hypervisors allocate system resources as specified by administrative constraints imposed on virtual machine guest operating systems and applications. The action of actively monitoring and allocating specified resources generates a lot of work for the CPU. Hypervisor system clock ticks and resource controls, combined with host VM guest operating system ticks, simply prevent CPUs from resting and therefore saving power.