MirageOS is a project of Xen?
Not sure what you mean by that…?
The Xen Project is a Linux Foundation Collaborative Project (or so its webpage says).
The Xen Project hosts Mirage OS as one of its unikernel projects.
Xen was one of the original targets for Mirage OS, though there are now quite a few others.
Why for Unikernel the Ocaml programming language needed?
OCaml is not needed for a unikernel. It happens to be the language that Mirage OS used.
(And a mighty fine language it is too, or so I am told… ;)
Why not other programming languages?
Why not indeed — there are many other unikernel projects that use other programming languages. Even C these days I hear.
http://unikernel.org/projects/ has a (likely incomplete) list.
I’m not sure where exactly is the source of that figure — but my gloss on it would be that the "unikernel apps” (labelled “VM”) each contain both the code you’d usually think of as being the app, plus the functionality for which an app would usually rely on a shared kernel as statically linked libraries. As a result labelling both identically as “Kernel” might be slightly misleading as the code represented could be different in each case (eg., the lefthand app might have a network stack but no filesystem, the righthand app might need a filesystem but no network).
