Operating System Design/Kernel Architecture/Exokernel
An exokernel is a type of operating system where the kernel is limited to extending resources to sub operating systems called LibOS's. Resulting in a very small, fast kernel environment. The theory behind this method is that by providing as few abstractions as possible programs are able to do exactly what they want in a controlled environment. Such as MS-DOS achieved through real mode, except with paging and other modern programming techniques.
LibOS's provide a way for the programmer of an exokernel type system to easily program cross-platform programs using familiar interfaces, instead of having to write his\her own. Moreover, they provide an additional advantage over monolithic kernels in that by having multiple LibOS's running at the same time, one can theoretically run programs from Linux, Windows, and Mac (Provided that there is a LibOS for that system) all at the same time, on the same OS, and with no performance issues.