Plex86 is no longer a universal x86 virtual machine instead it's current goal is a implement enough of the architecture to run the Linux kernel and applications which run on top of it. Kernels to be booted with Plex86 are compiled without most of their hardware support and IO systems and as a result are very simple.

Plex86 is now a lightweight virtualization program for the x86. It no longer intends to implement the dynamic translations techniques which are needed to support the entire x86 ISA. As a result, Plex86 no longer runs arbitrary binary OS's. The aim of the new-look plex86 is to provide a virtualization architecture for running many copies of Linux (and presumably other Open Source Unix clones) much like you can do on some of IBM's serious server hardware.

See for more information.

Disclaimer:I am in no way involved in the Plex86 project, I just use their software, so I might be wrong.