The past participle of "strive" is "striven" or "strived."

He was a king. He had been Striven to achieve the throne by his uncle.

Both "has striven" and "has strived" are correct. "Has striven" is more commonly used in formal writing, while "has strived" is acceptable in both formal and informal contexts.

I strive to explain right now. Indeed, I am striving to explain. Yesterday, I strove to explain. In the past, I have striven to explain. If I keep this up for long enough, I will have striven to explain. The verb to strive means, basically, to try. If you noun the verb, an ancient practice that some purists hate despite the fact that we have always done it, the strivers are those who strive, and the striven could be those who have striven or who have been the object of striving. I have never heard the noun phrase "the striven" before.

Both "I have strived" and "I have striven" are grammatically acceptable past participles of the verb "to strive." "I have striven" is less common in modern usage, but both are correct.