They are different mainframe configurations.There is term called VM (Virtual machine) ..i.e IBM's Virtual Machine (VM) operation system for S/390 and zSeries processors.
IBM's z/VM is a high-performance, interactive, multiple-access operating system for IBM zSeries and IBM S/390 servers. It provides a unique mix of services in support of interactive users and client/server environments, and its implementation of IBM virtualization technology provides the capability to run full-function operating systems such as Linux and z/OS as "guest" systems.
VM has been providing the capabilities to "virtualize" the complete S/370, S/390, and zSeries architecture allowing a single physical processor to run multiple guest operating system simultaneously with each guest thinking it has complete control of the system.
z/VM is the successor to IBM's VM/ESA, VM/SP and VM/370 operating systems. VM has provided a stable, robust and easy to manage platform for virtualizing IBM mainframe hardware.