Search threads for Cv and Kv for this forum. Kv is the 'european' (metric based) flow coefficient, and as Cv has to be measured for each single valve or construction.
Values are normally given from manufacturor.
Kf and K? Depends on context and orogin of definition! I have seen vseveral variations of use of thesse abbreviations / letters. Also: as alomst any strainer can have a number of different inside 'filter' (number or diameters of holes, thread thickness and idstance etc.) you have to go to the 'source' to find the correct answer.
And as stanier says: fluid, flud condition and filter condition?