Validation is making sure you have data backing up that something works the way you say it works. For example, a kill step is validated if you have data showing that in your process with your parameters at your facility does, in fact, kill pathogens by more than 5 logs, or whatever your limits are.
Internal auditing is more of a verification that everything is being done the way it is supposed to be, that metal detectors are being checked as described in your program, that the maintenance department or production departments are documenting what they are supposed to and when.
For a HACCP plan, I would say you are verifying the plan when you and your team go through it for an internal audit. You make sure everything is in order, and records are kept. Validation would be for specific steps throughout the HACCP plan, such as CCPs, to prove they work. The HACCP team can review effectiveness of the plan, for example, if you find metal in your product and your HACCP plan does not address metal anywhere, then you need to reassess the risks, but I don't know that I would think of it in terms of validating the plan. An internal audit is usually a checklist to verify that everything is in order for when you get a third party audit or customer audit, etc.