Kauravas were more powerful than Pandavas in many areas.
What was the main reason that Kauravas hated Pandavas?
Did Pandavas hate Kauravas too?
both @noob and @Ishika are correct in their statements but just to make things a little more clear, the father of Kauravas (Dhritarashtra) and the father of Pandavas (Pandu) were brothers wherein Dhritarashtra was the elder brother but he was blind since birth that is why it was decided that Pandu would become the next king of Hastinapur. At one point of time Pandu renounced his kingdom and started living in the forest, giving the responsibilities of the kingdom to Dhritarashtra (this is a very abridged version of the story, a lot of incidents take place in order for this to happen).
Duryodhan was the eldest son of Dhritarashtra whereas Yudhishthira was the eldest son of Pandu (need to note that Yudhishthira was elder than Duryodhan). The Pandavas as children grew up in the forest having no idea that they were entitled to a kingdom, on the other hand Duryodhan grew up in the castle believing that he was the righful heir of hastinapur. Once the Pandavas returned to the Palace of Hastinapur, the Yudhishthira was made the new King of Hastinapur and hence the basic reason of rivalry between the Kauravas and Pandavas.
A lot of things did happen over the course of time from being children to adults in the lives of both the sides to increase the animosity but I guess that's for a different time.