Comparing anyone to Cannavaro is always difficult due to the sheer significance of that player. He captained his country to World Cup-success, which means that comparing Barzagli to him will seem weird to some people.
Cannavaro was also arguably the best defender in the world for a vast portion of the timespan between 2004-2006. And as for the years preceding that era, he was always a very good defender.
However, I can still understand why some would compare the two, because the truth is, Barzagli has been putting in Cannavaro-esque performances for us on many occasions. Does he have Cannavaro's significance yet? No. Will he ever? In all probability he won't. That would require Italy to win the next World Cup with him having a truly wonderful tournament.
But I don't find it silly to point out that Barzagli has reached an exceptionally high level of defending, a level that can be compared to how Cannavaro used to play at his best.