KVM and Green Computing
A ran across this article today from Tom Henderson that draws attention to the fact that most existing hypervisors (ESX, Xen, Hyper-V) do not support frequency scaling and therefore are not very eco-friendly.
This is partly true. There has been some recent work in Xen to add deep sleep state support and I believe even some work on frequency scaling. It is certainly not true though that virtualization and power-consciousness are at odds with each other. KVM is able to leverage all of the work that's been invested into Linux to manage power wisely. Good power management does not cause any sort of performance drop. Reducing the performance of your workload is only going to make the machine run longer and consume more power.
The reason most hypervisors don't support power management is that it's very hard. When inventing a new Operating System, there's a lot of things you have to focus on before you can even start looking at power management. Again, we see the benefits of using an existing Operating System for virtualization.