Can I install multiple VMs on one machine without a host OS and be able to switch between them? This scenario would have a mix of Linux VMs -- several Ubuntu VMs and another distribution – and there should be a way to toggle between them with a hotkey.
If you want to switch between different VMs, you need something the VMs have in common to handle it -- that cannot be in the VM. The functionality has to be provided by an outside mechanism. The outside mechanism cannot be a thin hypervisor like VMware ESXi because it doesn't have any embedded methods for accessing the VM consoles.
Is there a reason why a host OS can't be used? Is it because of the overhead that is involved in running a host OS? The only solution that I can think of, particularly since you've only mentioned Linux distributions, is by running Linux containers.
In Linux containers, there are different VMs that run on top of the same kernel. As you don't install separate kernels for every VM, it is lightweight. It's just one kernel, running different instances of an operating system on top of that kernel.
From one of these instances, you could run Virtual Machine Manager to switch between instances of these operating systems. The one limitation is that the operating systems need to be very similar. As far as the Ubuntu VMs you mention, that probably wouldn't be a problem. But, depending on the Linux distribution you use, there may be too many differences from the Ubuntu VMs. It could be a challenge to get it to work the way you would like.
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