The Cvs for manual valves aren't standardized. You can do some looking around the manufacturer's sites and get an idea but if it's critical, you'll have to look it up for the specific make and model you have.
The top quality valve manufacturers do publish Cv values for their manual valves, and Googling as advised above will get you the information. But if you are doing a preliminary design and the valve hasn't been specified or bought yet then you will be looking for generic data and not ultimate accuracy. A web search for "globe valve Cv" is unlikely to find generic data, but might find you data on a manufacturer's site for a specific valve. You will be more likely to find generic data if you search for "globe valve K-value" or "globe valve resistance coefficient". Just be careful that there is also a Kv value that is the metric equivalent of Cv and very different from the resistance coefficient (K-value).
Anyway, once you have found a K-value (which should be about 5.5 for a 6" globe valve) you can convert it to a Cv using the formula
Cv = 29.9 x d2 / [√]K
where d is the pipe ID in inches
Plugging 6" into this formula with a K of 5.5 gives a Cv of 459, which corresponds quite well with the value of 446 from my Kitz catalog.
But once youhave the details of the actual valve it is always best to get the manufacturer's data. For example, if you select a guided plug valve the Cv could be significantly less than the estimate above.
Bear in mind that Cv values published by manufacturers tend to be empirical, i.e., they are the observed values of a sample set. Large manufacturers that push out thousands of units of the exact same valve can do this, since they produce and reproduce the same valve over and over. That's why they can give the Cv value. Also, producing the same valve over and over offers end users reassurance that they can replace one valve with another of the same brand/model without disturbing flow values.
...need I remind you all that both Cv and K values for any valve are by definition (look up Kv and Cv!) actual measured values: the measured throughlet of water under certain, defined, conditions?
The answer for finding of Cv for a given manual valve are then
a) mesurement as given in definition / certified measurement methodes.
b) compare or transfer from empirical or measured data from other, similar valves as adviced in this string.
