I have created users with respective their permissions from Magento admin panel. Now I need to login these users from Magento frontend...
(Kindly note that customer login is different part. I need separate button on Magento frontend for user login)
I get your meaning: You created an admin user for login admin panel, but you want to perform the login action on frontend(i.e. Customer login page), not on admin login page.
Actually Magento 1.9 has a module to perform your task already. You can use Magento_CrossAreaSessions to perform your works. So that you can login as admin on customer page, and vice versa.