VMware ESX 4.0 Update 1 and Later and vCenter Server 4.0 Update 1 and Later

vSphere Resource Management Guide : Managing Memory Resources : Administering Memory Resources : Understanding Memory Overhead

Understanding Memory Overhead
Virtualization of memory resources has some associated overhead.
ESX/ESXi virtual machines can incur two kinds of memory overhead.
ESX/ESXi memory virtualization adds little time overhead to memory accesses. Because the processor's paging hardware uses page tables (shadow page tables for software-based approach or nested page tables for hardware-assisted approach) directly, most memory accesses in the virtual machine can execute without address translation overhead.
The memory space overhead has two components.
For ESX, the service console typically uses 272MB and the VMkernel uses a smaller amount of memory. The amount depends on the number and size of the device drivers that are being used.
Overhead memory includes space reserved for the virtual machine frame buffer and various virtualization data structures, such as shadow page tables. Overhead memory depends on the number of virtual CPUs and the configured memory for the guest operating system.
ESX/ESXi also provides optimizations such as memory sharing to reduce the amount of physical memory used on the underlying server. These optimizations can save more memory than is taken up by the overhead.