Down Hole Safety Valve (DHSV) is a safety valve placed below the seabed this is the most important barrier in the well.
The vale is operated at surface, it is opened with hydraulic pressure via a control line down to the valve. Wireline retrieveable types have the control line going down between the sealing elements of the valve.
This is a fail safe device, if the hydraulic pressure is lost, the DHSV closes.
They are either tubing retriveble(permanent) or wireline retriveble.
Principle of these are ball valve or flapper valve.
Other types ball valve: