What speakers should I get?

So I have a 2006 Honda CRV. I replaced my front speakers with Polk DB651 speakers. I have a Kenwood radio not sure the model, I want to change my back speakers but I have no idea what to put there. I know nothing about the different speakers. Should I just go with another pair of Polks?

I would go with Polk to match.

There's no advantage to having the same brand front and rear speakers. Polk is a good brand and there's certainly nothing wrong with getting another pair, but many brands have comparable quality speakers.
I always suggest visiting some stores in your area and listening to a variety of speakers. Maybe you'll find something you like the sound of a little more, or something significantly less expensive that sounds just as good.

Pioneer, Rockford Fosgate, Kicker, Alpine and several others have decent quality inexpensive speakers.

If you like the polks, get more polks.

Your CRV can take 6.5" speakers in the rear doors or 5.25" speakers with adapter plates. The DB651 was a very good choice and another pair would be fine, but you might want something with a little more high range clarity, so I'd look at the 6.5" Alpine SPR-60 ($110/pair) or Pioneer TS-A1375R in 5.25" size ($80/pair)

Here s another direction: CRV is a small vehicle so two front speakers should be plenty. I would invest in a subwoofer to fill the bottom level, or at least some 6.5s that go way down. However, without an after stock amp it can't be done. Hope that helps. I've got a Fiat 500L with a full Hertz/Audison system that I had professionally installed, so I only speak as an enthusiast that knows the basics of car audio.