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Clarified 'modified OS' a bit, remove excessive Capitalization, use ''italic'' in →Para-Virtualized Machines
'''Virtual machines''' emulate some real or fictional hardware, which in turn requires real resources from the ''host'' (the machine running the VMs). This approach, used by most ''system emulators'', allows the emulator to run an arbitrary ''guest operating system'' without modifications because guest OS is not aware that it is not running on real hardware. The main issue with this approach is that some CPU instructions require additional privileges and may not be executed in user space thus requiring a ''virtual machines monitor'' (VMM) to analyze executed code and make it safe on-the-fly. Hardware emulation approach is used by [http://www.vmware.com/ VMware] products, [http://parallels.com/ Parallels] and [http://www.microsoft.com/windowsserversystem/virtualserver/default.mspx Microsoft Virtual Server].
=== Para-Virtualized Machines Paravirtualization ===This technique also requires a VMM, but most of its work is performed in the Guest ''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.xensource.com/xen/ Xen] and [http://user-mode-linux.sourceforge.net/ UML].
=== Virtualization on the OS Level ===
