Below are various notes regarding Virtual Machines as I introduce myself to them.  Most notes will likely be relevant to Xen and/or RHEL 5 -- my first reference source is:

http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/pdf/Virtualization_Guide.pdf

The initial testbed is the former stargrid01 machine, which is about five years old as I write this - hardly cutting edge.  Basic specs:  dual Xeon 3.06 GHz with Hyper-Threading, 2 GB of RAM (though 256MB seems to have disappeared).  It has Scientific Linux 5 installed, with virtualization support (xen-3.0.3-80.el5 for instance).

Two types of virtualization:

• Paravirtualization:  faster(?), but requires the guest OS to support it, so not as flexible.
• Full virtualization:  requires CPU extensions, which began to be included in Intel processors around 2005 and in AMD processors in 2006.  Thus most of the machines available to me for testing will not support full virtualization.  Virtualization extensions are "vmx" on Intel CPUs and "svm" on AMD processors (see /proc/cpuinfo) - but in some cases they may be disabled in BIOS.  In other words, if the machine is from ~2006 or newer, but the extensions aren't present - check the BIOS to see if they can be enabled!  The PAE instruction set should also be present (pae is much older and essentially ubiquitous in any systems still in service)

Useful commands:

• 'xm' -- Xem Management.  Sub-commands follow, eg "xm list"
• 'virsh' -- similar to xm, with sub-commands to follow
• 'virt-manager' -- starts a GUI VM administration interface