What are the forces that make water to flow within the xylem from the roots to the leaves?
Water enters the roots because of the root pressure and a water column is maintained within the xylem from the roots to the leaves. The most significant factor that makes water ascend is transpiration, mostly in the leaves. As leaves lose water by transpiration their cells tend to attract more water creating suction inside the xylem. The cohesion property of water that keeps its molecules bound (one pulls the other) by hydrogen bonds helps the process.
Plant Physiology - Image Diversity: xylem conduction