This is a picture of the previous example.
The host MySQL database is mounted inside UML on /mysql. The OS/400 JVM is represented inside UML as a process. Maybe Java is fed into the 'jvm' stdin and output grabbed from its stdout. Whatever Windows resources are desirable are either mounted as filesystems or represented as processes. The Apache running inside UML has local, native access to Linux, OS/400, and Windows data, resources, and applications.
So, what OS is it running. Obviously, it's inside a Linux virtual machine, but it also has elements of another Linux, OS/400, and Windows.