Can we have a OS Virtualization and User virtualization?

I'm not sure I agree with all of Arunkumar's list, but it is definitely a good start.

Server virtualization involves hardware virtualization. There are different levels of that as Arunkumar suggests. IBM systems (not Intel based) have the concept of Logical Partitions (LPARs) where there is a hypervisor that allows you to divvy up the physical hardware into multiple partitions - each in turn will run a full instance of a supported operating system. For example, one POWER based system w/4 cores and 16GB of memory could be subdivided into four (4) unique systems - each with 1 core and 4GB of memory - if divided equally. There is also the ability to make the resources of the LPAR dynamic such that they can grow/shrink depending on their needs or the needs of other running LPARs. Each of the LPARs could run AIX, PowerLinux, or i5OS - so the single hardware could be running three OS virtualizations.

Not sure what you mean by User virtualization. In a multuser OS - like *ix and Linux, does each unique login by the same user mean virtualization? I ask because each individual login by the same user is treated separately, the only issues being shared resources like $HOME directory. With some "tricks", multiple instances of the same user can share process space/memory, but normally they are not.