I've moved my virtual machine introduction to it's own web page so I can just find my links and notes quickly.
Strictly speaking, QEMO is not a VM environment, but a CPU/hardware emulator. It is used to provide a virtual environment.
Xen is the closest you can get to a true virtual environment these days, such as what is used on the IBM platform. Unfortunately, the host OS still needs to talk to the goofy PC hardware, but after that all the I/O and other interfaces are truly virtual.
libvirt is a virtual virtual machine tool. Essentially it is a library to provide a common interface to a flotilla of virtual machines. This lets you use the same command repetoir with most any virtual machine and hypervisor. One strong advantage of this is that you can swamp out hypervisor implementations with little impact.
GuestFish, or guestfs is a shell for mounting and doing I/O on virtual machine disks from a hypervisor. It will let you modify the disks of actively running VMs, so you need to be careful
Open vSwitch (named after VMware's vSwitch) is a virtual switch used to take care of network traffic to VMs on a hypervisor.
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