Pharo, like its siblings Cuis, Squeak, and NewSpeak runs on a VM called Open Smalltalk. It is written in a stylized version of Smalltalk so can be debugged with the tandard Smalltalk tools (i.e. you can have the modified VM you're working on load and run an image - but your VM is running in a debugger!)

Then when you're happy with your revised VM, you can spit out portable C code, compile it, and BAM you've got a production VM! Most plug-ins and primitives are also written in the Smalltalk subset (called SLANG), so they will likely be similarly portable.

There is also an FFI for interfacing beyond the standard model.