I am doing a minor variation (the original theme was in G major), so does that means that all the chordal harmony has to be minor as well?

Assuming we are talking about sample exam questions here rather than "real composing", I reckon traditional minor harmonies will be expected. Perhaps you could add some melodic minor scale passing notes for a bit of variety.

Good luck!

No, this is for a variation for my AS composition.

Yes, the harmony needs to be minor too.

In this variation, the tonic chord (i) would be minor, i.e. G minor, but the dominant chord (V) would (normally) be major, i.e. D major: this will give a perfect cadence V-i. It depends a bit on the voice leading etc, but having a proper cadence (V-i) is important at the end. You will need accidentals to achieve this, as the key signature will have changed from one sharp to two flats.

The above assumes you haven't modulated, and simply have a change of mode from major to minor.

P.S. J.S. Bach's 'Goldberg' Variations are in G, your own choice of key, and would provide a wealth of inspiration should you feel in need of such input.

Thanks Storini. Yes, at this point I am just having a cadence point but will be modulating later