Barca meet United in the final at Wembley on Saturday night in a repeat of the 2009 showpiece in Rome.
Although Pep Guardiola's side got the better of Sir Alex Ferguson's men that night, Xavi's respect for his rivals remains.
"What Manchester United have done over the years has so much merit," he said.
"They have been playing for many years at an extraordinary level, they have won many trophies, they have been competing consistently, getting to Champions League semi-finals and finals.
"They are on a spectacular level; all the footballers are at an extraordinary level."
Xavi has been particularly impressed by the longevity of veterans Paul Scholes and Ryan Giggs, as well as manager Ferguson.
"What Ferguson has done, what Scholes and Giggs have done - to be competing like they have, year after year, has a lot of merit," he said.
The midfielder believes United possess excellence all the way through the side.
"They haven't only got talent," he said. "They work well in defence, they are compact, they have many variants in attack.
"If they score a goal they close up at the back and play on the counter - they have a lot of tactical options."
Xavi was linked with a move to United earlier on his career but insists he is happy to have stayed at Barca.
"I'm in the Champions league final for the third time with this generation and I wouldn't change it for anything," he said.
Team-mate Andres Iniesta admits he is also attracted by English football and that beating United would mean a lot.
"Football in England is special," the midfielder said. "It's a country with great players, the stadiums are nice, and the culture, the people - how they live football and the matches - mean that it's always special to beat the English."
Iniesta was doubtful for the 2009 final, but an intense recovery programme ensured his presence in Rome.
"That final had its own situation and characteristics," he said.
"All the work I did in the two weeks before the final paid off because we won the Champions League."
The midfielder has enjoyed an injury-free campaign this time around and says he is feeling good ahead of tomorrow's clash.
"I'm in good shape," he said. "The fact that I can compete at 100% with no niggling injuries is great - I hope we can win it again."