Is it possible to different types of OS on different VPS hosted on the same main server? and virtualization is based on hardware level?

Lots of that varies. Some VPS's are emulated at software level, others at hardware.

You can run nearly any Linux distrobution on Xen and OpenVZ, you can run Windows and Linux on VMWare and Virtuozzo.

This is possible but not recommended.

I think this is possible with Xen, but I'm not sure. I don't think it would be recommended by any providers either.

Do you mean that you want to run just one virtualization technology (such as Xen, VMWare or KVM) but have the virtual guests run different OSes? If that is the case then certainly, this is one of the core benefits of virtualization. Each VPS is isolated after all, so what one is running should not affect any of the others.

On the other hand if you wish to run multiple virtualization technologies such as Xen and VMWare together they would almost certainly get in each others' way, and more to the point this doesn't seem to be a very sane thing to want to do.

Having said that you might be able to get away with mixing para-virtualization and hardware based virtualization as para-virtualization is an OS level technique, not a hardware technique. It is perfectly possible to run KVM on a machine running an OpenVZ capable kernel. I have tried this during testing but don't know how stable the combination truly is.

If you could clarify what you want to achieve it would be quite helpful.

Also,

If you mean that you want to run different distrobutions of Linux eg. run one VPS with CentOS and another with Ubuntu, it's most certainly possbile. It's just not possible to virtualize both Windows and Linux VE's on the same node.

I know that Virtuozzo can do it as long as the kernels are the same... i.e. Red Hat / CentOS / FreeBSD, etc..

It's not possible with windows on virtuozzo though.

I think VM Ware is probably still the best for running different flavors.