Here's what I've gathered from my research:
1. desmethoxyyangonin - "mystery kavalactone". Generally the scarcest of the 6 kavalactones in most kava root and not much is known about its effects or pharmacology.
2. dihydrokavain (DHK) - "heavy" kavalactone. Effects include sedation, muscle relaxation; moderately long-lasting.
3. yangonin - "heady" and "edgy". Stimulation, energy and euphoria; can cause anxiety in some people. Known to be a ligand at the CB1 cannabinoid receptor, but whether this plays a role in subjective effects is unknown. Also shown to have the most potent MAOI activity of the 6 kavalactones.
4. kavain - the main heady kavalactone. Kicks in quickly and for a short duration, effects are mental relaxation, euphoria, feelings of well-being. Shown to bind to the GABA receptor but at an atypical site (not the GABA nor benzodiazepine binding site). Known to block voltage-gated sodium and calcium channels.
5. dihydromethysticin (DHM) - very sedating and long-lasting, can cause nausea. The likely culprit behind the next-day sluggishness after a large kava session. DHM is generally pretty low in noble kavas since it's generally the most undesirable kavalactone and has been selected against over the hundreds (or thousands) of years of kava domestication.
6. methysticin - another "mystery kavalactone" at least for me. I don't really know what it contributes to the effects. Known to act a sodium channel blocker/antagonist (along with kavain).