I am owner of http://www.ravimobiles.com/ based on Valentines day. my topic does not match to domain name, is there any problem for my website in gaining rank?
No, it won't affect your rank. It will rank just the same as long as your content is targeted, which it looks to be. However, in my experience, it does give you a little "temporary" boost, but again, it's mostly on the content. Typically people buy targeted domains because they already know the niche of their site and how they'll develop it. In general, that's why they rank. I will say this, if I saw your site on the serps with that domain, I wouldn't think it would be paired with valentines day related greetings, etc. It "can" also hurt the resale value of the site, if it gets big and you want to sell it. If you just started the site, find a promo code, pay the $2-8 for a new domain, and transfer your site. You just gotta ask yourself, is my site worth it?