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This technique also requires a VMM, but most of its work is performed in the ''guest OS'' code, which in turn is ''modified'' to support this VMM and avoid unnecessary use of privileged instructions. The paravirtualization technique also enables running different OSs on a single server, but requires them to be ported, i.e. they should «know» they are running under the hypervisor. The paravirtualization approach is used by products such as [http://www.xensourcexen.com/xen/ org Xen] and [http://user-mode-linux.sourceforge.net/ UML].