When I use FIWARE FI-LAB, it's possible to use GE instances already deployed on FI-LAB (https://account.lab.fiware.org/) In order to deploy my application with back-end and front-end, do I need to integrate my Virtual Server or can I use Virtual Machines of FI-LAB?
You can use the virtual machines that you have deployed with the FIWARE Lab. In fact, If I understand you, you can deploy virtual machines with the apache tomcat already installed on it besides with other virtual machines with installed mysql and then you only need to install there your application.
I recommend to take a look to the presentation Setting up your virtual infrastructure in FIWARE Lab