Server virtualization is what you generally see used with data centers, web hosting companies and many other small and medium businesses.
Operation System Virtualization: This sort of virtualization mainly uses a “container" approach wherein virtualized clients (which have applications running in them) interact with a well-known and common operation system. As an example, think of a web-hosting company which offers a “Shared Hosting" package. They usually make you feel as if they have a server that hosts your website and appear as if this server just manages your website and nothing else. However, the truth is that this very server also manages to host many other sites as well. This form of virtualization comes in handy with multiple clients (computers) that use the same applications and use the same operation systems.
Hardware Emulation : As the name suggests, hardware emulation is all about using standard virtualization software (also called a Hyper Visor) to form a emulated hardware environment (Called VMM -- Virtual Machine Monitor), for guest operating systems to function on. The way this differs from an operating system virtualization is that you have guest operating systems (Windows, Mac or Linux) machines interacting with a VMM individually as a consistent package -- making it one unit, virtually. This package can be cancelled, shut down when not required or even migrated. The VMM in turn interacts and resides on the Hyper Visor (Software like VM Server, etc.). This kind of system allows for use of various operating systems and different computing systems for each one of the virtual machines, unlike the operating system virtualization which has severe limitations to variety.
Para virtualization: Para Virtualization is the much sobered down, budget-minded virtual traffic police cop. Instead of having anything to with code-heavy software like the Hyper Visor (the virtualization software) or just emulating hardware as in hardware emulation, we now have a wafer-thin code and a relatively light software program which just allocates resources to the requesting guest operating system. Much like a traffic cop, it allocates the limited hardware resources --like server space -- to the guest operating systems which are needed at that point and limits access from non-active guest operating systems, thereby saving resources.