I'm not claiming this is the ultimate authority but this makes sense to me:

"There seems to be some judgment required when forming the possessive of a noun that already ends in "s." Usually, add "'s" (another syllable): "bus's" or "boss's." However, quite often it would seem to depend on just how you personally say the possessive. My dictionary says that both Jones' and Jones's are correct. Another source says that boys' is correct, as it is normally pronounced as one syllable. In most of these cases, it would seem that you can get away with either apostrophe-s (usually preferred) or just the apostrophe (also acceptable)."

So I would think Cavins's, because it sounds like a separate syllable to me.

The wonderful thing about English is that it is very fluid, so you may find someone else will disagree with Mr. King and me, and I won't argue the point very hard.

I have to tell you the truth, I often rewrite the sentence to avoid the situation, so I will refer to "the house that belong to Calvins" rather than "Mr. Calvins's house."

But that's the coward's way out. ;)