[stanley]

I would say 'seldom' is interchangeable with 'hardly ever'. 'Hardly' can be used in several other ways, that do not mean 'seldom', eg: with 'any' as in 'hardly any English people speak Chinese' or meaning 'do not' as in: 'I hardly think that's relevant'
To simplify, I would say 'hardly' means 'almost no' or 'almost not', whereas 'seldom' means 'rarely' or 'hardly ever'
Clear as mud?!