Xen is a virtual machine monitor that allows many instances of an operating system to share hardware together. It employs the use of paravirtualization but with support for existing ABIs and multi-application OSes. Memory is statically partitioned between each guest OS domain to provide strong isolation, although the guest OS may request additional memory pages or return unused ones to Xen. Xen uses disk I/O rings to handle requests from the guest OSes. I/O requests placed by the OSes may be reordered before Xen handles them and places the completed ones back in the ring.
XenoLinux performs similarly to Linux in regards to context switch times, file system latency, various process benchmarks, and so on. The differences, such as in the execution of fork, exec, and sh, are due to verifications Xen must make of page table updates since the guest OS runs at a lower privilege level than the Xen hypervisor.
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