User-mode Linux (UML). Virtualization at the OS-Layer, basically "Linux inside Linux"
User-mode Linux (UML) permits Virtualization at the OS-layer, otherwise known as "Linux inside Linux" similar to Solaris Zones ("containers").
Linux "guests" are defined and managed by a single "UML" enabled-kernel.
There are several Separate Kernel Address Space (skas) modes available, which segregate the main "UML" enabled kernel from the "guest" instances.