I would need to say that because of the ignitor in the XXV burning corn would not be a good idea. The moisture that you get from the corn may mess up the ignitor. Harman has a PC45 corn stove that has a completely different ignitor because of these reasons.

I have seen people burn a mix of 1/3 corn to 2/3 pellet in a Harman pellet stove. I have heard up to half and half, but I would probably lean more toward the 1/3-2/3 mix. Other than making it tougher for the stove to light, corn is really tough on metal. In laymans terms it eats metal. Both the stove and the pipe. There is corn pipe which is made a little sturdier but the outcome is the same over time. Harman's corn stove also has a stainless steel burn pot to help lengthen the life due to the stress of corn. Corn also does not have as thorough of combustion. The high moisture contributes to this as well. There is an agitator in Harman's corn stove that helps to break up the inevitable clinker(s) to help make it easier to clean.
If you want to burn corn I would recommend the PC45, if you already own an XXV I don't think I would use more than 1/3 corn and expect that you will have increased maintenance issues and requirements, you will shorten the life of your pipe/stove, and may have difficulties with the unit lighting correctly.