Can I use a personally owned aircraft in the EU for PPL flight training?
Can I use my own C152 to gain my PPL (EASA) with a qualified instructor?


It doesn't matter who the owner is, so yes. As long as you pay to keep it in an airworthy condition and fuel her up, she's all yours.


  Is this a considered a commercial flight?


The fees the instructor is getting is for teaching you to fly, so no. Also you'll be the one who's paying.


  Can you define a 'training flight'?


Definition (my own, Google not helping): Any flight with a CFI on board, or approved by your CFI, with the goal of getting a higher certification.

Which in the broadest sense, every single flight is a training flight. Don't let the over confident pilots fool you, every flight is indeed a lesson.


  Is hour building a commercial flight?


Any flight builds hours. The answer is no if you're training, in your own plane or otherwise.


  If I use my private 152 how about insurance?


The EU is big. Check with insurance companies. But most likely it's no different than normal insurance. What if a flight school owner (who is not a pilot) decided to learn in one of his planes and the instructor is one of his employees, would that change the insurance terms? Nope.

Last paragraph actually answers all questions.

Fly safe!