By implication they mean similar things, and can be used together. If someone ‘keeps to’ him or herself, then you might infer that he or she is reserved, or introverted. Is someone seems introverted, e.g. they don’t speak much in social situations, you might conclude that person is reserved and might tend to ‘keep to himself/herself.’ Here are some examples -
‘He’s quite introverted, he tends to keep to himself’.
‘He seems quite reserved’
‘I think that’s just his manner - he’s quite introverted’
‘Did you meet the new neighbour in flat 6?’
‘No. She tends to keep to herself’
So they aren’t exactly interchangeable, for example in the last example the person might be avoiding others for reasons other than her being introverted so you couldn’t really say ‘she’s introverted’ or ‘reserved’ here, but they do all have similar meanings.