What's the feature of each ballet company?Royal Ballet,ABT,Paris Opera Ballet,Staatsballett Berlin,La Scala,Bolshoi,Kirov,NYCB,San Francisco Ballet...
I especially interested in the difference between Bolshoi and Kirov.I feel Bolshoi and Kirov are so different,but don't know how to describe...So I hope to listen to your opinion.

bolshoi lover, that's quite a large question! If you search through past Ballet Talk threads, you'll probably find answers in many places. Here are a few places to start:

Imperial and Vagonava styles
Bolshoi Style
Royal Ballet style
Kirov/Mariinksy style
Royal Danish Ballet style
NYCB style
La Scala style

You'll also find forums devoted to each of the companies you ask about under the European Ballet Companies and American Ballet Companies headings. You might want to add comments or ask questions on one of those existing threads.