You do have to remove the toilet tank to replace the toilet flush valve.
The tank of the toilet will need to be unbolted from the toilet bowl to access the large nut on the toilet flush valve to remove it.
You should also use new toilet tank bolts with new seals otherwise it will leak.
Without removing the toilet tank you won't be able to remove the old toilet flush valve.
The difference between a flush tank and a flush valve is flush valves mix air with water to achieve more pressure while flushing to provide a cleaner water closet bowl.
Flush tank toilets feature a tank that holds water until a flush is initiated, at which point the water is released to clear the bowl.
Flush tank toilets have a fill valve that is connected to the water inlet that uses a float system to allow water to turn on and fill the tank and then the float shuts the water off when the tank is full.
A flapper in the toilet lifts up when you press the flush handle and allows water out of the toilet tank into the bowl to flush the toilet.
Flush valves usually use water pressure instead of a normal flapper to force water into the toilet bowl for a better flush.
Some toilets also use pressurized water tanks that force water out into the bowl instead of filling a tank with water which also saves on water.