If user tries to go to welcome page, without authentication, he shall be redirected to login page,
but when I run the application, only login.xhtml page is accessible without authentication.
After logging in all the pages are accessible.
I want only welcome.xhtml page to be accessed after authentication.
One of the reasons why I recommend using the J2EE standard security system is because you don't have to do convoluted things like this when you use it. Although the main reason is because, as I've said too many times, it's because I've never run into a user-designed security system that was actually secure. And in your specific case, ANY request from a logger-out user to a secured URL would automatically redirect to a login without a single line of Java code required.
I would not use JSF for this kind of task, in any event. It would at best only be secure in cases where each and every submitted URL with security implications was a JSF URL, and that's rarely the case for me. Requests to servlets to create reports, requests to JSPs to create XML files, stuff like that would not go through JSF and therefore would not go through a phase listener. A servletlistener is a better place to put stuff like this.
Sometimes the only way things ever got fixed is because people became uncomfortable.