hello arun, to begin with, i am an amateur, and have just started a project. i have tried my best to answer.
MVC pattern would be the answer to your question. leave all the business logic to model(POJC, EJB), presentation logic to jsp, and control(servlet) as a mediator between view and model. Though you can call model from view, it' a bad practise. Think of jsp as a dymnamic HTML page that clients see(ofcourse, that what they are developed for). with dynamism comes the scripting. don't use secripting in jsp, use ELs, tags etc.,instead.
so, to epitomize, use all the three (jsp, servlet, and POJC or EJB).