This is a very difficult question. 势头 means the trend or tendency of the development of something. 经济发展势头很好。我感觉股市的发展势头不妙。这位歌星在国内势头正劲。今年中国的经济发展势头明显放缓了（放缓means become slow）。 声势and 气势 are very similar 声势usually means some movement's impetus . 这场运动的声势正在增强。这场斗争声势浩大。 气势 also have the mean of movements' impetus. 这场运动气势磅礴。 气势 also can be used to express someone is very angry . 她气势汹汹地冲到我的办公室。他气势汹汹地向我扑来。 气势汹汹地here is adv. i hope my explanation is helpful.

If someone gets special or powerful background,then we can talk:这个人来势（或者来头或者势头）不小啊!!! If we are attending some big ceremony or festival,then we can describe:节日里，声势浩大，响声隆隆。 If we are shocked by some fabulous landscape or Architecture,then we chant:啊！我从未见过如此气势磅礴之景！

势头 is spoken chinese. 声势 prestige and influence 指名声威望，气势 气势 spirited atmosphere，指表现出来的力量，威视