There is no real equivalent as far as I know, Hefin is a masculine name and Hefina is the feminine variant.
It's from the Welsh word for "summer": haf, which also stands in its own right as a popular girl's name: Haf.
Is the girl's name always pronounced "hahv"?
As I understand it, the season itself is often pronounced simply "hah" -- is the double pronunciation (hah / hahv) also possible for the name? (I'm guessing not.)
As you probably know, final f is often dropped in colloquial Welsh: diwethaf, nesaf, cartref and so on can be said diwetha, nesa, cartre. The general rule is that this f-dropping occurs in polysyllabic words all over Wales and in some monosyllabic words in the north, so:
formal: diwethaf; informal south & north: diwetha
formal: cyntaf; informal S & N: cynta
formal & informal S: haf; informal N: ha
formal & informal S: cof; informal N: co
There are exceptions like araf doesn't drop in the south despite being polysyllabic; first person singular future verbs can keep or drop the f in the south depending on the dialect: bydda(f), wna(f), a(f) etc.; braf doesn't drop the f because it's a borrowed word.
But whereas the common noun haf might be realised as haf or ha, the proper noun Haf is always said with the f. I guess it's because Welsh speakers think of haf and Haf as being separate things.