Wow! I had to do some research on this one as I had never heard the term exo-kernel before. An exo-kernel is a new operating system research area. A micro-kernel architecture, like the Carnegie Mellon Mach operating system, implements a minimal kernel that does just device drivers, virtual memory, message passing, and thread scheduling. An exo-kernel operating system is simply a paper thin abstraction of the basic hardware, with everything else done in user mode. As with almost all research done in white tower academia, both micro-kernel and exo-kernel designs are completely impractical in real-world development. The true micro- and exo-kernel operating systems would take up way too much memory and resources to be anything close to practical. Windows 2000 is basically a modified micro-kernel that borders on a monolithic kernel.
This was first published in February 2001