This emulation layer in turn uses the
real hardware resources from the host. This enables the emulator to run a
guest operating system without any modifications, as the guest OS can use
the hardware resources by going through the hardware emulation layer,
instead of the real hardware. The VMM executes the CPU instructions that
need more privileges than are available in the user space. This approach is
followed by products such as VMware (http://vmware.com/), Microsoft
Virtual PC (http://www.microsoft.com/windows/products/winfamily/
virtualpc/default.mspx), and Parallels (http://www.parallels.com/).
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Introduction
[ 6 ]
Paravirtualization: There is no hardware emulation. The operating system
that runs on a guest needs to be a modified version that is aware of the
fact that it is running inside a hypervisor. This cuts down the number of
privileged CPU instructions that need to be executed, and as there is no
hardware emulation involved, the performance is much better and closer
to native speeds. This is the technique used by Xen (http://www.cl.cam.
ac.uk/research/srg/netos/xen/) and User-Mode Linux
(http://user-mode-linux.sourceforge.net/).
Operating System level virtualization: Each guest instance is isolated and
runs in a secure environment. However, you can execute only multiple
instances of guests that run the same operating system as the host. If the
host operating system is FreeBSD, you can run only multiple instances of
FreeBSD.
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