Types of virtualization

There are basically four types of virtualization, Hosted, Binary Translation, Paravirtualization, and Hardware Assist.

• Hosted: This type of virtualization uses a base operating system to run the physical computer and the hypervisor manages access to the physical resources through the operating system. The base operating system is normally Windows or Linux, but there are hosted virtualization versions for the Mac. VMware Virtual Server and Microsoft Virtual Server 2005 are examples of a server-based hypervisor using a hosted design.
• Binary Translation: This type of virtualization has a very thin operating system below the hypervisor. The hypervisor captures all system calls for hardware resources and translates the virtual calls to physical calls. By translating all system calls, each virtual machine is completely isolated from the underlying hardware. VMware ESX server and Microsoft’s Server Core are examples of this type of hypervisor.
• Paravirtualization: This design of hypervisor allows some specific system call to be passed directly to the physical resources. The remaining system calls are still translated before passing to the physical resources. In a true paravirtualization hypervisor, small pieces of the guest operating system are modified to modify the dangerous kernel operations. These changes are picked up by the hypervisor and translated to the physical resources. Some less disruptive hardware calls are allowed to pass directly to the physical resources.
• Hardware Assist: This type of hypervisor leverages the benefits of the paravirtualization design and takes it a step further by adding specific CPU calls from the guest virtual machines. This allows for an even thinner hypervisor and increased performance of the virtual machines. Both Intel VT and AMD-V are examples of hardware assist in a paravirtualized hypervisor. Commercial versions of this type of hypervisor can be found in Citrix XenServer, Microsoft Hyper-V, and VMware ESX 3.5.