Illia

From my experience and opinion, cold hardy usually refers to bananas who's corm and p-stem can tolerant temperatures in the high 20's for brief to moderate periods of time.

Agreed with the rest though, Cavendish are far from hardy (mine started yellowing just above freezing) and are best if protected or moved indoors during the winter. Protection must be a serious case though, as with your zone it takes a lot to heat things up to the temperature you want. If you've got room, time, and a little money a greenhouse is a great idea though. But warning, greenhouses spur on tropical fruit growing addictions, haha! Non-heated ones usually boost the winter temps to a zone above where you are, heated ones depend on heating but should keep it above freezing. There are methods of heating one that don't require electricity, some less and some more effective.