I would only talk about the 2 hypervisors used for virtualization in detail (type 1 and type 2 hypervisors , i hope u will gain a good understanding on them.).If you have any question at all feel free to ask.
What is an hypervisor:
Intermediary service that allows the user to create virtual machines on top of it. The operating system which is not virtualized is called the host operating system which is readily available. The virtual machines are called guest operating systems
optimization: The computer resources are divided and dedicated for the virtual machines created. The virtual machines are guests as the hardware is not directly allocated and that it is still running on top of an operating system .The main operating system is called the host .
Type 2
This hypervisor can be called software hypervisor as it is installed on top of the operating system.
The hypervisor will virtualize hardware ,often para-virtualization(hardware functionality not modified but runs isolately ) where the resources are allocated by the operating system.
Slow
No special hardware required
ex: VM-box ,VM-player , Oracle VM ,VMWare workstation
Type 1
The hypervisor is directly installed on top of hardware .On top of the hypervisor would the host o/s and guest operating systems work .The hardware is now directly controlled and coordinated by the hypervisor not the host operating system. The hardware abstraction layer acts as a hypervisor(but only has basic features) but the hypervisor which is installed for MS server operating systems is an advanced form of hypervisor .
ex: Hyper-v ,Vsphere
___________________________________________________________________
Sources: