Can a muslim marry a christan women, and both donot want to change their religion. Under what circumstances is it possible for both to marry.

I say that if you want to be with each other then yeah, but it's going to need some sacrifices from both of you. I'd have a compromise between you to decide about it, that way you both get what you want... :)

Yes, this is a halal marriage, it is aloud. But I would encourage you to know for sure if he wants to marry you, dating is not aloud in Islam, and you mentioned he is your lover, to have a relationship as is understood in Western countries is not aloud for Muslims. Ask him about it .

Try to know if he wants to marry you for real? Has he met your family already? Do you know his family? Do they accept you? just a few questions to think about it.

Best is that you choose the same path for your life.
The path that looks more real and logical to both of you