penalty, interest and fine paid/ payable under direct taxes is not deductible., however penalty which is compensatory in nature & which is paid for breach of a contract or statue is deductible. if something more is payable than that , then it is not deductible.
Interest on Service Tax paid is a deductible expenditure. The reason being that interest payment is compensatory in nature, while penalty or fine is an infraction of law AND HENCE NOT DEDUCTIBLE.
if any interest is paid on delayed payments like service tax, excise duty or vat etc is deductible expenditure. Interest on late payment of tds or income tax u/s 234 a,b,c is not deductible and same is added back in computation of income.