Traditionally, applications have run directly on an OS on a PC or a server. Each PC or server would run only one OS at a time. Thus, the vendor had to rewrite parts of its applications for each OS/platform they would run on. An effective strategy for dealing with this problem is known as virtualization . Virtualization technology enables a single PC or server to simultaneously run multiple operating systems or multiple sessions of a single OS. A machine with virtualization can host numerous applications, including those that run on different operating systems, on a single platform. In essence, the host operating system can support a number of virtual machines (VM) , each of which has the characteristics of a particular OS and, in some versions of virtualization, the characteristics of a particular hardware platform. The VM approach is becoming a common way for businesses and individuals to deal with legacy applications and to optimize their hardware usage by maximizing the number of kinds of applications that a single computer can handle . Commercial VM offerings by companies such as VMware and Microsoft are widely used, with millions of copies having been sold. In addition to their use in server environments, these VM technologies also are used in desktop environments to run multiple operating systems, typically Windows and Linux.The virtual machine monitor (VMM) , or hypervisor , runs on top of the host OS. The VMM supports VMs, which are emulated hardware devices. Each VM runs a separate OS. The VMM handles each operating system’s communications with the processor, the storage medium, and the network. To execute programs, the VMM hands off the processor control to a virtual OS on a VM. Most VMs use virtualized network connections to communicate with one another, when such communication is needed. Key to the success of this approach is that the VMM provides a layer between software environments and the underlying hardware and host OS that is programmable, transparent to the software above it, and makes efficient use of the hardware below it
