They are synonyms. Loathe is a bit more subtle than Hate.

'To loathe' is much stronger than 'to hate'. It implies deep-seated, simmering hatred. 'Hate' is also a much more common word.

I'm not native, but I'd say that 'loathe' is much stronger than 'hate'. This reminds me of the Grinch. http://www.youtube.com/watch?v=vT7sOOc4DOc "Hate, hate, hate. Double hate. LOATHE ENTIRELY." :)