Kvitova also beat the Italian en route to her previous title -- her ninth -- in New Haven on the eve of the U.S. Open.
Despite her improving game, Kvitova downplayed her latest success.
"I am not thinking like this is a turning point or turning tournament for me," said Kvitova, who won Wimbledon in 2011. There are "a lot of great players in the top, and that's hard to beat them. I'm still thinking about my game. That's the priority for me."
She had won her previous three matches with Errani, ranked one spot better than the Czech at No. 7, but this was their toughest match yet.
Errani lost the first four points and her serve, and Kvitova easily won the first set.
The Czech appeared to lose her focus in the second set, committing consecutive double faults and winning just one game. Kvitova had eight double faults, which she attributed to a shoulder problem that has been troubling her since Doha the previous week.
"Yeah, I know, that is a lot of double faults, and it wasn't really good in my mind to (be) thinking about it," she said. "But I played another top-10 player. That's OK."
Changing strategy to pressure her opponent, Errani charged the net and surprised Kvitova with drop shots.
"She was hitting so strong, making winners everywhere," Errani said. "I had to change something to try to win the match, and I did in the second set, tried to go more on the net, be more aggressive. On the baseline she was playing unbelievable, so it was very tough."
Kvitova admitted the second set depressed her. What changed in the deciding set was staving off break points in her second service game. Boosted, she went on to put away Errani.
"I think I had my chances in the third set, love-40 on many games, and I lost them," Errani said.
Errani also reached the indoor final in Paris this month. She and Roberta Vinci, whom she beat in the semifinals, won the Australian Open doubles title.