What distro is XCP-ng based on?

I'm curious to know, can you tell me which distribution, or "distro" for short, is XCP-ng based on? It's important to understand the underlying foundation of a technology like this, especially when it comes to ensuring compatibility, stability, and security. I'm eager to learn more about the technical aspects of XCP-ng and how it's built, so any information you can provide would be greatly appreciated.