Asrin, I think Visitor visa is the only relevant visa that can be issued to ur parents. Generally 5 years visa is issued for visitors but their stay should not be over 6 months in each visit and again they can visit multiple times. So what you can do is apply them for visitor visa. They will come here and when thier stay comes to be near 6 months then take them to a trip to Canada or Mexico. While returning back they will be given another 6 months of stay permit. You can do this on and on if you want to lengthen their stay.