From Xen

Mini-OS is a tiny OS kernel distributed with the Xen Project Hypervisor sources. It is mainly used as operating system for stub domains that are used for Dom0 Disaggregation. But it is also used as a basis for development of Unikernels, having been instrumental in the formation of multiple examples including ClickOS and Rump kernels.

As of March 2015 mini-os now has its own git tree. Previously (release 4.5.0 and earlier) it was part of xen.git in the extras/mini-os subdirectory.

Also See