LaLiga champion Barca has been linked to Neymar, which left Camp Nou for French giants PSG in a world-record €222million deal in 2017.
PSG sporting director Leonardo confirmed Neymar – who failed to turn up to the first day of pre-season training – is available for the right price.
Xavi played alongside Neymar before moving to Qatari side Al Sadd in 2015 and the 39-year-old said: "He would be an incredible signing.
"But I see it as being difficult, honestly. I think it's very difficult.
"We have seen so many things that I thought I'd never see though, such as PSG paying €222 million, so we'll have to wait.
"There's no doubt that he's one of the best players in the world, but let's see what Barcelona do."
While Neymar's future is uncertain, Atletico Madrid star Antoine Griezmann is reportedly set to join Barca.
France international and World Cup winner Griezmann – who has a €120million release clause – will leave Atletico after five years and Barca are favourites to sign the forward.
"He's a great player, a world star," Xavi added. "He made a difference with Atletico Madrid and with France. He's an extraordinary player.
"It's still not official, but it seems like he's going to Barcelona. I think it'll be a great signing."