An exokernel is an evolution from microkernels, as they export a virtual machine that securely multiplexes resources among mutually distrusting applications.
The exokernel philosophy tends to export as few abstractions besides the basic hardware abstractions as possible.
Low-level operations commands are then stored in libraries which are outside the kernel and loaded on-demand.
The most advanced exokernel based OS is Xok which performances, when coupled with the Cheetah web server can be up to sixteen times better than a similar NCSA/BSD server. For more information, please visit : http://www.pdos.lcs.mit.edu/exo.html