"What would be your dream job?" would usually have a qualifier. "What would be your dream job if you could choose any job in the word?" "What would be your dream job within your company?"

There is no difference in meaning. 'What is your dream job' is a more likely to be used.

You can use both as they both relate to the future.

Almost exactly the same, except "would be" is the passive voice and "is" is active. Passive voice is generally more conversational, and less pushy. Active voice is more formal, professional, and assertive.

They are both fine! The second one is seen as more formal and less common.