I know that "Gewitter" means "Thunderstorm" and "Ungewitter" means "Violent storm", Could we say that Gewitter is a strom that have thunderbolt but Ungewitter may have thunderbolt?
Well "Gewitter" is something where you have thunder and lightnings, so it would be a thunderstorm in English. But a "Ungewitter" is an other form from "Unwetter" which could also be a thunderstorm, but more likely just bad weather with heavy winds and rain.
So a "Gewitter" is always a "Unwetter", but a "Unwetter" not always a "Gewitter".
But while speaking German "Ungewitter" is not that common, we mostly use "Unwetter" or just say there will be bad weather ("schlechtes Wetter").
"Ungewitter" is a word for "Gewitter", not used anymore (I never heard it before. Duden and several other dictionaries say it's an old german term). Usually we speak of a Gewitter or Unwetter.
Duden defines "Unwetter" as follows [translated from german]: "Very bad, stormy weather, usualy with heavy rain [and thunderstorm]. Its intensity causes damages."
While "Gewitter" is defined like this: "Storm ((in original it says even "Unwetter")) with lighting, thunder [and rain or similar]"
A Gewitter is a thunderstorm and Unwetter is a bad, stormy, weather condition in general, with or without Gewitter.