what about switching between hibernated OS stored on SSD ?
probably that soon is fast enough for what you had in mind ?
The reboot takes too long, even if the hard drive were as fast as ram.
You have a strange definition of "simpler". Since devices have to be initialized, which is what takes all that time when an operating system boots etc, transferring ownership of device between two "running" operating systems would require the device be frozen and shut down for the first OS and then initialized and "melted" for the second OS by the "simpler" hypervisor.
You mentioned hybernation, there is a reason that takes time.
Further, if anything changes in the state of your file systems or devices then your hybernation wake-up would fail.
So what you need to do to acheive your goal can be done, but not "instantly". Go get a RocketDrive(tm) or similar [e.g. a "hard disk" made entirely of RAM and a battery] and put your hybernation targets (windows hyberfile.sys, linux primary swap, etc) in partitons on that drive. The set up a good smart boot loader (like grub). Make sure none of your OS instances "share" any local (not NFS etc) partitions of any sort. Then set up hibernation hot keys.
Time to switch is going to be "freeze-reboot-pick-os-melt", but this is not "instant."
It's not like the inventors of these systems just put all that hibernation time in there to annoy you, that is work that has to be done. period. So you either partion your hardware or you hibernate your OS. There is no magical third mode.
"Since devices have to be initialized,"
This is an arbitrary assumption. Hibernation has a purpose other than switching o/s's, by definition de-hibernation starts from a shut down computer. And the little hardware driver development I did 20 years ago tells me initialization is not required if you are starting from a valid state because each device has a state held by internal registers that can be saved and restored at will. Remains to be seen if the same applies to today's pc's. Maybe it's ok if a video frame buffer is left unsaved, because it will be painted with new data immediately.
"Further, if anything changes in the state of your file systems or devices then your hybernation wake-up would fail."
File systems are ok as long as each o/s does not mess with the other's system partition, which can be made harder by turning one at a time into a hidden partition. And if you're in the middle of modifying something on a data partition, might as well be banned from switching over until all data files are closed.
Something close to instantly switching between a linux host and an xp host is now possible. Colinux runs both hosts in parallel.
A few cpu cycles will be stolen by the host you are not currently using. Should be ok.
You can't switch a physical l machine fror one OS to another - you have to dual boot. However if you have a Windows verison of WS and Linux Version of WS you can copy a VMware guest machine from one to another - Winddows to Linux and vice versa using the ntfs-3g driver. I do this all the time and it works fine. You should the WS program that you have copied the VM - which of course yo have - the default. But you still have to switch between Linux and Windows OS by dual booting. Perhaps you can do this with 2 physical machines - havfen't tried it ye However I have used Hyper-V for something similair - on 2 machines of course. If you find a way to do this on 1 machine please let the world know how you did did it.