I know this is quite an old question, but for others with a similar question, I thought I would add my understanding of Kern surname background. I am of German (Alsatian, actually) Kerns, and have heard a possible etymology between kern and <i>kernel</i> in German and Dutch, meaning that Kerns are corn/grain farmers.
There is a second Kern surname which is the Irish Kern from McCarron, Curran, etc., which comes from the word meaning warrior.
And finally (although this seems to be rarer), there is an English locale Kern which may result in residence/origin surnames of Kern.
So based on surname alone, both of those are certainly possible sources of the name.