“User Mode Linux (UML) allows you to run Linux kernels as user
mode processes under a host Linux kernel, giving you a simple way
to run several independent virtual machines on a single piece of
physical hardware. Let’s take a look at UML and how it can give you
more bang for the hardware buck, or make it easier to debug the
kernel.“Under UML, each of the virtual machines can run its own
selection of software, including different distributions of Linux
and different kernels. This gives you the ability to have
completely customizable virtual machines that are isolated from
each other, and from the host machine. Among other things, you can
use this technology to secure systems by containing
vulnerabilities, to give developers and sysadmins private sandboxes
for development and testing, and to debug problems in kernels using
familiar userspace utilities, such as gdb…”