A real historical figure, Xerxes inherited the vast Persian Empire from his father Darius. Though Xerxes was successful in controlling his Empire and its economy, he failed in conquering the Greek city states.
In ancient paintings and statues found throughout the Persian Empire, Xerxes is depicted with long dreadlocks and a long braided beard. His statues are impossible to distinguish from that of his father, Darius, so obviously they are not exact representations. Also, they feature him killing horned lions.
Xerxes was likely very intelligent as he was generally successful at running the Persian Empire (at least from a purely political point of view). He is also known to have devoted vast resources to building large palaces and monuments in his honor, and making a point to have them bigger than that of his phenomenally successful father. While this may be an indicator of his personality, it is also worth noting that the construction of these buildings and the splendor they brought to the land were great economic stimuli to the economy and also helped to secure his reputation as a powerful super-being, something which was understandably important in holding together his enormous empire.
The Persian Empire worshiped a powerful sun god and, as was customary at the time, the emperors were believed to be divinely appointed. Like his predecessors, Xerxes's religious claims implied he may have god-like abilities, or at the very least could easily invoke the power of his God. He was widely treated as more of a deity than as a man. However, he allowed near total religious freedom in the non-Persian lands his Empire occupied. As long as taxes were payed and soldiers were available for his wars, he did not interfere in their cultural beliefs, even if they directly contradicted his theological argument for divine power.
As part of his appearance as larger than life, Xerxes was adorned with priceless jewelry and majestic apparel. When traveling, Xerxes sat in a throne atop a large stone slab that was carried by at least a dozen servants.
Persia was a polygamous state, and it is likely Xerxes had many wives, possibly numbering in the hundreds.
In 300, Xerxes portrayal is fairly accurate to the limited historical records. Xerxes is shown as a skilled political leader who eagerly displays a vast array of personal wealth and power as well as the threat of divine ability. He wears layers of gold jewelry and rides atop a throne on a stone slab carried by servants, both of which are well-documented historical facts. Unlike the monuments Xerxes had built for himself, which depicted him with dreadlocks and a braided beard, the movie Xerxes is shown to be bald and clean shaven. He is also shown with facial piercings and with no tunic under his golden chains, both of which are highly unlikely. However, these are certainly meant to be a stylistic way of amplifying his extravagant appearance. Xerxes true height is unknown; the movie assumes he was very tall indeed.
The movie's biggest liberty is to exaggerate Xerxes claims to divine power, having him specifically state he was a God and requiring worship from all of his subjects (an oversimplification). The film suggests Xerxes believes all of his claims, though an argument could be made either way.
Xerxes' caravan is shown to be full of lascivious dancers and female prostitutes, but it is suggested that Xerxes may have strong homosexual feelings. There is little evidence on the real life Xerxes' sex life, but he probably had many wives.
At the end of the movie, Xerxes has a facial scar on his left cheek, courtesy of the Spartans.