The first one. Chavez's pen. It's a singe person so the apostraphe has to come after the last letter and before the 's'.

Chavez's pen. The special rule exits for words ending in 's' because of the possibility of a plural possesive. There is no possibility of a plural ending in 'z'. One more point - Chris's car is also correct according to some, since Chris is singular. The important thing here is that if you used the "s's" form with a singular ending in 's', you should do so in the rest of the document. The entire text should be consistent.

It may be worth pointing out here that "Bridget Jones's Diary" was a grammatical joke, which was only corrected at the end. (Talking about the film.) The s' only applies to that letter, not similar-sounding ones. In speech, if you say " Bridget Jones' ", you are still pronouncing it as "Brid-jet Joen-zez", adding the extra syllable for the plural.