Real Kingdom Hearts
In ancient times, people believed that light was from Kingdom Hearts. It was guarded by the X-Blade. Warriors vied for the light. They made keyblades which are replicas of the X-Blade. The clash shattered the X-Blade into twenty pieces, seven of light and thirteen of darkness. Without the X-Blade, Kingdom Hearts is swallowed by the darkness.
Organization XIII's Kingdom Hearts
When Xemnas was created, he used his yellow colored Kingdom Hearts to find the heart of his natural self, Xehanort. Xemnas wanted to fill his comrades with the exact same heart. Most of the members were not fit for the task. The only ones fit were Xemnas, Xigbar and Saix.