ARE you using Linux kernel version 2.6.21+ in your VMs ?
As of Linux kernel version 2.6.21, the kernel no longer has a fixed
1000Hz timer tick. This will (in theory) give a huge power savings
because the CPU stays in low power mode for longer periods of time
during system idle.
BETTER STILL are you using XEN VMs without any VGA card emulation !
The Intel graphics driver used to have a bug where it set up the
hardware to generate an interrupt (called VBLANK) each time the screen
finished refreshing (typically at 60Hz or 72Hz). In normal 2D mode,
this interrupt isn’t actually used. PowerTOP unveiled this bug and the
current Intel graphics driver no longer has this behavior.