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Server virtualization uses specialized software called a hypervisor to run multiple server operating systems on a single physical server. A few hypervisors run on top of a base operating system such as Windows Server 2008 or Linux, while others such as VMware vSphere or ESX run without a separate operating system.
Hypervisors take advantage of the fact that many server hardware systems have multiple multi-core processors and enough RAM (up to 256GB), to handle many simultaneous systems. The hypervisor allows you to run multiple operating systems such as Windows 2008, Windows 2003 or various versions of Linux on the same box. Each virtual machine (VM) has a designated number of CPU cores, RAM, hard disk space and network interfaces available to it.
Some hypervisors allow for thin provisioning, where the OS will use up to a designated amount of CPU power, RAM, hard disk space and so forth, but only when needed. This allows for more VMs to run on a single hardware system.
What Server Virtualization products are available?
Server Virtualization products are available from several vendors, including Microsoft's Hyper-V, VMware's vSphere, ESX and ESXi, Citrix Xen Server and Red Hat's KVM. There are open source versions of hypervisors such as KVM available as well. Hyper-V is part of Microsoft's Windows Server 2008 R2, while vSphere, ESX and ESXi are dedicated hypervisors that don't require an underlying operating system. Xen Server and KVM run on top of a Linux kernel.
Pricing of hypervisors varies widely, from free products to thousands of dollars per physical system.
They also vary widely in their management capabilities, ease of installing guest operating systems and features, such as thin provisioning that allow for VMs to be installed that don't use server resources until they need them.