One question.. How can Professor Xavier is alive?

Rating: 10/10 (5 February 2017)

Hi all,
I have one question, How can Professor Xavier is alive after he was died in 3rd installment (X-Men Last Stand) by Jean. May be I have missed something or may be this is plot hole?
However, I loved X-Men movies but when I was watching this series second time then this question raised in my mind. Can anybody please clear this confusion ?? I think I surely missing something here as this can't be plot hole. If it is then it would be biggest mistake in movies for me.
My favorite character of course is Wolverine. I would love to watch logan in Cinema. He is just awesome.