Q:

Which class i need to use for custom authentication in symfony2

I am totally confused with the object oriented inheritance things
I have this scenario
Class  User
Then 
Class  Employee extends User   attributes username, password
then
Class   Owners extend  User
Each class has different attributes
Now all the login details will be in User table. 
But i am confused that suppose 

If i use class User for authentication and Employee logs in, then the object i will be having is of type User not employee
How will i get access to the attributes of Employee from that User object.  If iclick on edit then how will employee details gets loaded

A:

You need to use some kind of inheritance (single table or multi table, doctrine should return the correct type, even if you query on the normal User entity), and have each object return different roles depending on their type (if you want to limit their access). 
https://github.com/netmeansnet/NmnMultiUserBundle this can proberly be inspirational.