He was obviously enhanced by the system, the synergies with and the calibre of team-mate - "I play well by association", Xavi. But you can draw a parallel between Xavi's peak years, including his eventual decline, with the success and fall of both Spain and the possession game of Barcelona. Spain became the most successful international team of all time in winning three consecutive major trophies between 2008 and 2012 when Xavi was at his peak. That period of dominance started before he'd kicked a ball for Guardiola when he was player of the tournament at Euro 2008, in a system which was more fluid and direct than that what followed. And since he declined after 2012, Spain have been nowhere on the international scene.
At club level, the trajectory of Barcelona - peaking from 2008 to 2011, as one of, if not the greatest side of all - mirrors Xavi's peak. Once he declined, Barcelona weren't the same force and, despite their best efforts, couldn't replicate his role in their system. So much so that they had to switch to a more transition-based system to become successful in the Champions League again. And at both club and international level, even within those peak years, you can pinpoint the odd game where Xavi was missing and see the clear disjoint in the system and the reduced performance level that followed. Spain become notorious for losing their shit when he wasn't available, despite having a plethora of elite central play-making midfielders to call upon. Xavi wasn't just successful because of the system Guardiola developed. He was the system.