The trio will join Lazio, Bayer Leverkusen and Sao Paulo respectively, with the Portuguese and the Mexican initially leaving the club on loan until the end of the season
Pabon, 26, who only moved to the Mestalla in August from Mexican outfit Monterrey, has joined the Brazilian club on a permanent basis until June 2015.
Meanwhile, Postiga has sealed a loan deal with Edy Reja's side until the end of the season, with the Biancocelesti having the option to make the move permanent in the summer.
Similarly, Guardado will join Leverkusen on a temporary basis until June before the German outfit make the decision to bring him to the BayArena for good.
Both the Mexican and the Portuguese will travel to their new clubs for the formal completion of medical procedures before the deals are finalised.
Valencia are enduring a disappointing season in La Liga, with Juan Antonio Pizzi's side currently in 10th, 17 points off the Champions League places.