Most commonly z/OS, z/VM as a hypervisor or just native Linux(rhel, sles, Ubuntu or Debian)

Depending on what model of mainframe you got you can use up to 60 logical partitions which are firmware level virtual machines. and can extend this functionality using z/VM or just plain KVM in a Linux environment

At the raw metal level, zOS. But [you can run Linux on top of that](https://www.ibm.com/it-infrastructure/z/os/linux].

In addition to those already mentioned - z/VM. It's the grand-daddy of all hypervisors. z/VM can run Linux, CMS, DOS/VSE, z/OS, or even another z/VM (referred to as 2nd-level VM).

The other thing is that mainframes can be (and usually are) partitioned using software called PR/SM. It wouldn't be uncommon to have z/OS running in 1 partition and z/VM running in another.

z/OS