But problem is, when I search them on the Xing or Linked in, they get
the notification, will it make bad impression?
How far are you going here? If you are going to try to add them as a LinkedIn contact before meeting them then yes it would make a bad impression. On the other hand, if you look through it to get some information that may be useful in the interview then it could be good. Also, be aware of whether you are researching stuff on them or are you trying to have an informal interview before the interview that may well go bad.
If you sent them a letter declaring your undying love for them then it would likely make a bad impression yes as it would seem like you are obsessed and in need of medical professional help with this issue for another extreme example here.
Is it very unusual to contact with them before the interview?
I'd say it is unusual to contact them, yes. If you are merely looking at their profile on the site, this isn't the same as contact to my mind.
At the same time, if you are at a networking event and run into a future interview person then this can be quite good to have an opportunity to get to know the person outside of the office to some extent. Similarly if you run into someone that you had an interview a week or two ago, it can be useful to catch up on things.