Hi Roy,
I think that what you want is 'shut off', that will turn it off as if the power button was pressed, the OS will halt the machine, then you will turn it on as if you pressed the button and the OS will do the boot as usual.
Shelving will save the instance state in disk, so when you unshelve it will be as if nothing happened, but it does not use RAM from the hypervisor. I think there has been some issues when unshelving losing the state, so might not work 100% of the time. It will become 'shelved offloaded' eventually if not restored, and will need to be unshelved first before using it again.
Pausing is similar, but will store the VM status in RAM, so unpausing is faster, but uses RAM from the hypervisor while paused.
Suspend will let the OS do the suspension process, that should be similar to pausing, but letting the OS do it (instead of openstack) and stores everything on disk, so no RAM used.
If you need a quicker revert that waiting for the OS to boot, you can try to 'pause' it first for a second if anything should break right away, and if nothing breaks in 5min or so unpause and shut it off for longer (so it uses no resources).