Mac Private Clouds are made possible by a virtualization-providing hypervisor -- a mechanism that allows the physical resources of a bank of our Mac Pro or HP Blade servers to be pooled, so that they can be drawn upon much more dynamically and fluidly.
Orka is MacStadium's proprietary containerization solution for macOS. It effectively splits the difference between the two other options listed, in that it is powerful and easily extensible, but it is also simple enough that the learning curve isn't too steep.
A well trusted provider that offers less expensive, DIY virtualization options. It is worth noting that these options generally require a significant amount of technical expertise to use.
Built by Veertu, Anka offers out of the box macOS CI. The convenience of this service comes with a premium, as it is significantly more expensive than VMware's solution.
Find additional information for comparing the three here