Here’s why: When a virtual machine is running, inside the VM, the operating system maps out pages of RAM.  The hypervisor then addresses and stores those pages in physical RAM.  The address in physical RAM typically never matches the address that the VM’s operating system knows.  The hypervisor (specifically the virtual machine monitor or VMM) has to "translate" the fetching and updating of pages of RAM.  This puts an extra "tax" on the CPU and adds to virtualization overhead.  This can specifically be seen in workloads that perform very frequent page table updates.

The solution:  AMD and Intel continue to enhance their CPUs to offload some of the functions that virtualization provides in software today.  AMD and Intel started with their AMD-V and VT Extensions respectively and now continue on to the second generation of virtualization enhancements.  AMD’s is called Rapid Virtualization Indexing (RVI) and Intel calls theirs Extended Page Tables (EPT). What these functions do is manage the mapping of virtual pages to physical pages for fetching and updating memory so that the CPU does not have to do it in software.

Last week, VMware published a very interesting white paper on RVI and how it performs on some of the more demanding workloads (Specifically SQL, XenApp, and Apache).  You can find all the details and the white paper here .