Preventively means 'in a preventive way': you do something now so a bad thing is less likely later. A doctor might give medicine preventively, before the busy sick season, not after you already feel ill. A school might lock a gate preventively, even if nobody has tried to come in. Though actually, this adverb is not very common. Most people keep the adjective and say 'as a preventive step' or 'for prevention'. When you do hear preventively, it is usually in careful, medical, or official talk, not in friendly chat.