In the world of control valves the Kv value expresses the amount of flow in a regulating valve at a given valve position with a pressure loss of 1 bar. The special situation with a fully open valve determines the Kvs value. *The opening position of a valve changes the restriction or orifice in the valve thus changing the flow rate that can be past at a certain pressure.

The amount of flow at a lower pressure loss (Kv) can be calculated using the formula: 
Kv = Q / √ΔP 
where 
Kv: Kv value [m³/h] 
Q: Flow [m³/h] 
ΔP: Pressure loss over the regulating valve [bar] 

The Kvs value expresses the amount of flow in a regulating valve at a fully-open valve position and a pressure differential of 1 bar. The Kvs value is a special version of the Kv value, which indicates the flow at a given valve position and a pressure differential of 1 bar. 

The flow at a lower pressure loss can be calculated by rearranging the equation: 
Kvs = Q / √ΔP 
where 
Kvs: Kvs value [m³/h] 
Q: Flow [m³/h] 
ΔP: Pressure differential across a regulating valve [bar] 

See balancing and control valves here: ​https://www.flowquip.co.nz/shop/category/hvac-specialities-hvac-valves-93