The only resource settings I have seen per VM are:
Max. Memory (MB):
Priority: (Mine is defaulting to 50);
Processor Cap %: (Mine is defaulting to 100);
Those are the defaults I am seeing and I am not sure where those are set. Not sure what you mean by idle... Our VMs are either running or stopped (or transitioning between those two) and when no one is on them they use minimal resources depending on what listeners/services are running in the background. Generally the PVMs run with less resources than the HVMs & the HVMwPVdrivers are somewhere in the middle of the two.
AFAIK the above priorities & cap and maximums apply whether idle or not...
When there are no applications being run on the VM, basically not being used by any remote server etc. is when it is idle. There has to be some penalty memory consumption to avoid idle VMs and pause the VM temporarily or suspend to release the resources to the pool for other VMs to consume.
In our OVM 3.2.1 I see options to both Suspend & Resume VMs but haven't had a reason to try them until now.
I went in and suspended a VM and then resumed it.
Via 'xm top' I saw OVM release the RAM and cpu that the machine had when it went offline so a suspended machine doesn't appear to be using any resources when suspended (aside from taking up space on the disk). When it resumed & came up the Linux guest seemed functional (it had a message 'initializing CPU' message which I hadn't seen before but otherwise seemed fine).
I did this from the OVM GUI but it looks like the suspend/resume features are also available via the command line so they could be scripted:
The OS's them selves should also have the option to suspend or hibernate but I don't know if it is guest side initiated how that interacts with the host nor whether it releases resources... If the guest hibernate feature also suspends in OVM then you would fully release resources though I don't know how you would restart them via something like Wake on LAN in either case. Worst case you could have a simple web page that folk could click on to resume suspended machines that submits the resume command to the CLI.
Hopefully others have more experience with these features...