Parents can visit anytime, don't have to be a permanent resident. I had my in-laws visit me, when I was just 2 years in the country H1. All that is needed is affidavit of support (I-134) and some additional documents like bank statement, letter from employer that you are still employed, letter to the consulate stating the purpose of your visit and a letter inviting your parents to visit you with a reason.
They enter only on a B-1 visa, V-visa as I understand is for someone that has already filed for family based immigration benefit (485) and has passed a certain number of days (180 days I think, not sure).