The main difference between radial, mixed, and axial flow centrifugal pumps lies in their impeller design and flow characteristics. Radial flow pumps have an impeller that pushes fluid radially outward, resulting in a high pressure rise and low flow rate. Mixed flow pumps have an impeller that pushes fluid both radially and axially, resulting in a moderate pressure rise and flow rate. Axial flow pumps have an impeller that pushes fluid axially, resulting in a low pressure rise and high flow rate. Each type is suited for specific applications, such as radial flow pumps for boiler feedwater and axial flow pumps for irrigation systems.