I have made a beat of wiki research to try to have an educated answer to this interesting question. I think it's usefull as we see a lot of disrespect: Djokovic's fan think that Hewitt couldn't play tennis and dominated some nobody, while Hewitt's fan claim that Djokovic is so poor on grass that he couldn't take a set of him in prime to prime matches. I feel both views are stupid, and my research shows it.
1) The Head to Head
Novak leads the H2H 6-1, including 4-0 on grass. He defeated Hewitt twice at Wimbledon: in 2007 in a 4 very close sets (he won 3 tie break), and in 2010 in 4 close sets. He beat him at the olympics last year, but lost a set. He beat him very comfortably at Queens in 2008, Hewitt'worst year.
The lone victory of Hewitt happened in 2006, when Nole wasn't in his prime yet, and Hewitt no more in his either. The 6 victories of Novak happened since 2007, when Novak was in his prime or peak, and Hewitt wasn't.
2) Their 3 best year at Wimbledon
I compared their 3 best years at Wimbledon, based on their results. 2010-2012 for Novak, 2002, 2004 and 2005 for Hewitt.
Djokovic:
In 2010, Novak lost in 3 sets in the SF against Berdych. He beat Hewitt in 4 close sets in R4, who was the only though player he met (other opponents are Lu, Montanes, Dent, Rochus). He lost 2 sets against Rochus in the first round. At this time of the year, Djokovic hadn't beaten a single top 10 in his worst year by far.
In 2011, he lost a set against Baghdatis, Tomic, Tsonga, Nadal. Novak 2011 was something.
In 2012 he lost in 4 sets to Fed in the SF. He lost a set to Stepanek earlier, didn't really faced any really though opponent.
The others years since 2007, he only lost against good players: Nadal in 2007, Safin in 2008, Haas in 2009.
Hewitt:
In 2002 he won the titles against Nalbandian. He lost to sets against Schalken in the QF, otherwise straight-setted his other opponents, including Henman.
In 2004 and 2005, he lost both time against Federer, once in QF and once in SF. He took once sets in those matches. He lost one set to Moya in 2004, one to Dent and Hernych in 2005.
The other years, he lost in the first round to Karlovic in 2003, Baghdatis in 2006, Djokovic in 2007 and 2010, Fed in 2008, Roddick in 2009, Soderling in 2011, Tsonga in 2012: Even way past his best level, it took strong grass courters to beat him.
3) Winning percentage and grass titles:
In carreer winning percentage on grass they are very close. Hewitt is at .772% and Djokovic at .770%. Hewitt winning percentage was higher at the end of his prime in 2005, but not by much: 0.784%.
In titles won, Hewitt leads 7-1. Opposite to what a clueless young posters said, he didn't had it easy for these titles. He defeated a certain Federer to win Halle, another guy Sampras at Queens (twice), Ivanisevic, Pioline (former Wimbeldon runner-up), Henman, Nadal (!).
4) Conclusion:
Both players deserve a lot of respect. Djokovic is clearly the overall superior player, and while grass is his worst surface, it takes a stong performance to beat him here.
The H2H in his favor is difficult to interpret because all matches happened far after Hewitt's best year. Still, Hewitt wasn't easy to beat on grass in all these years, as shown by the names who defeated him. This H2H count certainly for something for Djokovic, but they were close matches.
The names Hewitt defeated to win his titles are quite something too: It includes the two best grass courters of the last 20 years, and some surface specialists like Ivanisevic and Henman.
Most of Hewitt's titles on grass happened in the early 2000, when the grass was faster and less bouncier. I think this is a big parameter in a fantasy prime to prime match. On fast grass, I think that Hewitt should have a clear edge. On slower grass, he would still have the edge, but it would be closer.