I have switched to .php but a few pages on my website are still in .htm IF I do a 301 redirect will I lose the page rank of those site and the backlinks? Thanks in advance
I did a similar thing on my site recently and switched all htm pages to php and I didn't lose any position in Google SERP. I can't say for sure but in my experience it didn't make a difference.
No, a 301 is the right way to do it - it's telling search engines that it's a permanent redirect and backlinks and PR belong there now.
301 Redirection is the right one to do. You won't loose your linkjuice of site with 301 redirection. Don't go for 302 redirection - there is more chance to loose the credibility of your existing site. For more information, please read this article..
301 redirect will pass the PR but there is no guarantee that the new page will get all the PR and ranking of the old page. Matt Cutts said something like that in his blog.