Related: GNUnix, Linux, nut, OS, seed, unix, user
A kern[a|e]l is the minimal policy code of an Operating System.
http://www.yl.is.s.u-tokyo.ac.jp/~tosh/kml >>Kernel Mode Linux is a technology which enables us to execute user programs in kernel mode. In Kernel Mode Linux, user programs can be executed as user processes that have the privilege level of kernel mode. The benefit of executing user programs in kernel mode is that the user programs can access a kernel address space directly. So, for example, user programs can invoke system calls very fast because it is unnecessary to switch between a kernel mode and a user mode by using costly software interruptions or context switches. Unlike kernel modules, user programs are executed as ordinary processes (except for their privilege level), so scheduling and paging are performed as usual.