>how the activation occurs once user session / data pages have been passivized ?
The details of how Pega implements certain feature is out of the discussion scope.
>how passiviation is been configured in pega?
There are a couple of prhelp documents describes how to configure passivation, Timeout passivation, for example.
>what are thing we have to check before configure this for our application and how to configure?
If your system doesn't have specific performance issue or high availability requirements, you don't need to care too much about passivation.
Typically, page passivation improves performance for production systems that might have 50 or more simultaneously connected requestors (not counting agents). For smaller systems (with adequate JVM memory), the benefits might be small or none at all.