The ex-Spain international was a youngster breaking through at the Nou Camp when Van Gaal was in charge at Barca and his first impression was not great.
However, he soon altered his view and grew to love the Dutchman - a feeling which does not yet appear to have materialised among Manchester United fans since he took over at Old Trafford last year, with Xavi admitting the 64-year-old is also "poorly valued in the media".
"After two days of training under him I thought, 'Who is this idiot?' After a week, I thought, 'He's right.' He'll always be in my heart, Louis van Gaal," Xavi, now playing for Qatari club Al Sadd, told ESPN FC.
"[He is a] good person, very good trainer
He has a very strong character but he's calm in the Premier League because there's less pressure.
"He's much calmer than when he was at Barca or Bayern Munich
He's honest, methodical and a real perfectionist.
"He's very demanding, he's strict and he wants to achieve the highest levels possible.
"He's a tough fighter, yet he put trust into me when I was 17 and 18 years old."
Xavi also has experience of being coached by Jose Mourinho, who worked for Van Gaal at Barcelona, and he had nothing but praise for the Portuguese, whose methods are under growing scrutiny at Chelsea.
"He was excellent in his three years at Barca," Xavi added.
"They said he was a translator
Rubbish
He was the assistant coach, someone who understood the philosophy of Barca and who shared many of the same characteristics of Van Gaal.
"He was very respected by the players
He trained us sometimes alone at Barca B and he was excellent.
"I'm surprised that he became known for another type of football, more defensive, because he wasn't like that with us."
Source: PA