My store is using X-cart. I have plan to migarate it to Magento. Could anybody tell me which migration tool is secured?
It depends on your shop and needs. Both solutions below only offer migration of catalog (and sales) data. Any other data like CMS pages etc you will have to write a script for yourself. I've used both options below with success several times
On highly customized shops (specially on the database) it's best to write your own migrations. You'll need to generate CSV's containing the BigCommerce shops products, categories and customers. For Magento I'd suggest using AvS FastSimpleImport which is a wrapper of Magento's Dataflow import (which is a bit wonky)
2. paid service
For simple, default shops I would go with Cart2Cart. I'm not in the habit of promoting paid services but as far as migrations go they provide a very good and reliable platform. You'll have to install a file in either shop which will connect the stores and transfer categories, products, customers and orders