“If you run multiple operating systems on your Mac in virtual machines, then you likely use either Virtual Box, VMWare Fusion, or Parallels Desktop,” Topher Kessler writes for MacIssues. “For the most part, when you set up a VM on your Mac with any of these solutions, it will run as a window that shows you the graphical view of the virtualized OS. However, if you use your VMs for servers instead of running desktop applications, then you can use a small trick to run them in the background, and thereby save both a little processing power and some desktop clutter.”
“By default when running VM software will show some virtual machine list window, where you can select your desired VM and launch or configure it,” Kessler writes. “However, when run from these interfaces, a display view to the virtual machine will show. Unfortunately, closing this view will generally suspend or shut down the VM.”
Kessler writes, “For the most part, you might not be too bothered by having a virtual machine window open, since you can hide or minimize it to keep it out of the way; however, if you want a more seamless and ‘headless’ approach for running your VM’s in the background, then the following approaches will do the trick.”
How to run ‘headless’ virtual machines in OS X here.