[ Thanks to Michael
Hall for this link. ]
“Xen and KVM are built into the kernel, and essentially
provide the “hypervisor” functionality similar to Vmware’s
virtualization engine. With libvirt, the library for controlling
virtual machines, applications can be created to interact with the
hypervisor and running virtual machines. XenServer, owned by
Citrix, is based on Xen, but adds a complete GUI and other
functionality similar to what you would find in VMware, such as
storage virtualization. XenServer is less expensive than VMware,
but it is far from free. When we refer to Xen, we are really
referring to Xen, KVM, and any other built-in Linux virtualization
engine controllable by libvirt, but not XenServer.“Indicators of maturity, for most VMware users, mostly relate to
centralized management. The GUI should be able to perform any
management function, and should be the primary way to manage a
cluster of virtual machines. While this is truly a handy design,
the unfortunate part of open source software is that this rarely
happens.”