Traditional virtualization model
Applications providing virtual hardware such as VMware, VirtualBox, or QEMU
Host Operating System
In this traditional model for virtualization on the PC, the virtualization software runs on top of the operating system, and provides a virtual machine into which we can install an operating system. That virtual machine has to look like a real machine to the guest operating system, but it is created by the virtualization software and may use clever tricks to access some native resources from the real hardware.
Xen uses a very different model, the hypervisor approach (see Figure 28-14).
The hypervisor approach actually has a long history, going back to IBM's mainframe systems that have used this basic method as a way of running multiple operating system instances for many years. But on PC hardware, this approach is a new one. To understand Xen, it is very important to appreciate the difference between the two models.
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