I'm not sure that we can compare KML and WFS.

KML is a format just like GML, JSON, CityGML, Shapefile, etc, but WFS is a service. For example WFS outputformat can be GML, Shapefile, JSON, CSV and so on, I assume also KML is possible.

We can compare KML vs GML, JSON, CSV, Shapefile and other formats.

I fully agree to Peteris Bruns. KML is spatial data file format. Whereas WFS is a service by OGC.

OK, I agree,. But KML is also format of OGC and could be easy accessible. With GMK it is more complicate. You can make it easy accessible as URL. And WFS is protocol used for accessing GML. But for practical purposes, you can both publish from the some servers as URL and also access with the same clients.

But probably right question is, what is better KML or GML?

In my opinion KML is more suitable for visualisation than GML.