Is it possible to run multiple OSs at the same time on a single PC without using any VM or third party tool?

Yes it is possible.

There was a project a couple of years ago by Phoenix Technologies called HyperSpace. They built an ultra small Linux kernel that ran in BIOS/UEFI for instant boot. The system allowed people to browse the internet and do some low level functions while the other full operating system/s were loading. The system I had a chance to see was capable of running and swapping operating systems in real time (Linux/Windows) on a laptop machine. I wasn’t able to perceive visual lag at all playing a movie in each operating system and swapping between them as fast as a possible. Sadly the tech didn’t really go further in consumer laptops after HP bought it in 2010.

HyperSpace (software) - Wikipedia

Was a pretty awesome piece of tech, didn’t get any information as to how it worked on the lower levels but I did what it was suppose to do.

As far as I know, it is impossible an any machine made today or in the past. You would need some sort of hypervisor. However, it is possible that hardware could be built with the ability to host multiple OSs at the same time without a a software hypervisor. I don’t think the advantages would outweigh the limitations and expense.

Short answer: No. An o/s takes over control of the hardware - you really can't have two of them running at the same time.

Longer answer: One can envision some special cases.

• Compatibility layers like Wine - allowing windows applications to run under Linux. Not quite two OSs running at the same time, but sort of - from an application point of view. In a sense that's what the Mac OSX/OS10 compatibility mode did, as well.
• In some cases you can run an OS as an application, under another OS - though it really won't be doing a lot of what an OS normally does.
• I expect that, with a LOT of modification, you could get different operating systems to run on different cores of a multicore processor. Virtual machine ISA extensions would probably help.
• There are some experimental microkernel OSs where each program essentially its own minimal o/s - so that's kind of like running multiple OSs, but probably not what you have in mind. Also, you still need some kind of hypervisor to mediate resource access - though I guess the hardware might be able to do that.

The closest I can think of is one of those unique cases where you could fit in two full systems in one pc case. You’d be running two systems off of one power supply but they provide a splitter/adapter thing.

The company that designed this case is Silverstone, if I recall correctly.

That’s about as close as you can go. No hardware is capable of supporting that in it’s bare bones. That’s why people use hypervisors. But they are still VMs.