It still exists: https://wiki.xenproject.org/wiki/Mini-OS https://wiki.xenproject.org/wiki/Mini-OS . But beware that this is no more than a small reference OS,

It still exists: https://wiki.xenproject.org/wiki/Mini-OS https://wiki.xenproject.org/wiki/Mini-OS . But beware that this is no more than a small reference OS, there's a massive gap between getting it to just boot and running real-world applications with it.