Perhaps that's a common enough name in Canada, but here in the US there would be no cannonical way to pronounce "Penhallow", so you'd have to ask the holder of the name.
There is perhaps a certian small subset of names that have an expected "correct" pronounciation, (eg: Smith, Brown, etc), but this is not in them.
Complicating matters is that it contains the word "hallow", which different English accents pronounce differently. I doubt it is consistent across all of Canada. For instance, where I live that word is pronounced with a short a and the "ow" as a long o for the sence of "to make holy", but more like a short o and the "ow" as either a short a or more like an "er" if it is in the sense of "a valley between two hills".
Your best bet is probably to either change the question to, "Canadians: If you were saddled with this last name, how would you chose to pronounce it?", or to just pick a pronounciation you like, and move on.