Getting started with User-Mode Linux
LINUX IN LINUX
User-Mode Linux feels like Linux because it is Linux. You’ll find a hundred uses for this fast and sensible virtual Linux system
The popular and versatile User-Mode Linux (UML)  creates a fully operatational virtual Linux system on a Linux host. UML has many uses in the Linux world. Developers rely on UML to test their applications without putting the host system at risk. Linux users run UML to experiment with kernel versions without having to worry about a new or untested patch. System administrators use UML to test system configurations. You can even run multiple versions of UML on the same host to simulate a network. What is User-Mode Linux? User-Mode Linux is not really an emulator, nor is it an API. The best way to explain User-Mode Linux is to start with a look at the role of the Linux kernel.