I have had my Clarity for just three months now and have driven only about 1500 miles, so I think the following observations are correct and useful, but if not, I hope someone will correct me.
1. ECON mode, normal mode, and SPORT mode, with a few exceptions noted below, are merely different maps of the position of the accelerator pedal to the amount of power requested. The same amount of press of the accelerator results in smaller, medium, and larger accelerations, respectively. Any one of these modes are independently compatible with your choice of EV or HV. (Secondary issues: SPORT mode retains the stage of paddle deceleration under more instances than in the other two modes. ECON mode more slowly reacts to increased climate control commands or seat heating.) .
2. HV mode, as do most hybrid operations, tries to optimize and maintain maximum possible charge in the high voltage battery consistent with gas economy from the engine. It uses a variety of options: making use of direct connection from the engine to the wheels via gearing, or indirect connection through the electric motor, and under certain conditions with having the engine supply charge to the high voltage battery, etc.
I found the best way for me to get used to the way the Clarity operates is by driving with the Energy flow screen displayed on the audio/information screen.