What’s the difference between a thread and a process?

Xen, an x86 virtual machine monitor which allows multiple commodity operating systems to share conventional hardware in a safe and resource managed fashion, but without sac- rificing either performance or functionality. This is achieved by providing an idealized virtual machine abstraction to which oper- ating systems such as Linux, BSD and Windows XP, can be ported with minimal effort.

Xen, a high performance resource-managed virtual machine mon- itor (VMM) which enables applications such as server consolida- tion [42, 8], co-located hosting facilities [14], distributed web ser- vices [43], secure computing platforms [12, 16] and application mobility [26, 37].

Successful partitioning of a machine to support the concurrent execution of multiple operating systems poses several challenges. Firstly, virtual machines must be isolated from one another: it is not acceptable for the execution of one to adversely affect the perfor- mance of another. This is particularly true when virtual machines are owned by mutually untrusting users. Secondly, it is necessary to support a variety of different operating systems to accommodate the heterogeneity of popular applications. Thirdly, the performance overhead introduced by virtualization should be small.

full virtu- alization has the obvious benefit of allowing unmodified operating systems to be hosted, it also has a number of drawbacks.

Xen is intended to scale to approximately 100 vir- tual machines running industry standard applications and services

Xen itself the hypervisor since it operates at a higher privilege level than the supervisor code of the guest operating systems that it hosts.

Throughout the design and implementation of Xen, a goal has been to separate policy from mechanism wherever possible.

Wiki

Xen Project runs in a more privileged CPU state than any other software on the machine.