Virtual I/O (VIO) server is a virtualization technology by IBM. A Virtual I/O server is a logical partition (LPAR) that runs a trimmed-down version of the AIX operating system. Virtual I/O servers have APV support, which allows sharing of physical I/O resources between virtual I/O clients.

See the PowerVM wiki for more in-depth information about VIO server and virtualization:

http://www.ibm.com/developerworks/wikis/display/virtualization/VIO

For more information, see the PowerVM Virtualization on IBM System p redbook:

http://www.redbooks.ibm.com/redpieces/abstracts/sg247940.html