Hypervisor VMs

Virtual machines are at the core of server technologies like VMware Inc.'s ESX Server and the open-source Xen virtual machine monitor. Both of these products offer servers that run multiple x86-based OSs simultaneously. Their approaches are slightly different variations of what are called hardware-level, bare-metal or hypervisor virtual machines. The intermediary software layer (called the virtual machine monitor or hypervisor) is between the OS and the hardware. The hypervisor gives all the OSs that are running the illusion that they are the only OS running on the hardware.

Running multiple OSs on one server platform offers several advantages. It makes it possible to more fully use the resources of very powerful servers, provide backward compatibility for legacy programs and partition applications to different OSs so they can't corrupt one another.

VMware uses transparent virtualization, which means that the OSs that run on the hypervisor do not need to be modified. Xen uses paravirtualization, which means that it needs to modify the OSs to make them run simultaneously on the hardware. Xen claims that paravirtualization increases speed and efficiency.

Hosted VMs