To minimize downtime involved in a migration using a full database backup, you can use a combination of a full database backup, a differential backup (manual method only), and multiple transaction log backups. By using this method, you will keep downtime to a minimum. This method commonly is used with medium to large databases where minimizing downtime is of paramount importance. To test your database in Azure before migrating a production system, either use the full database backup method or use this method to test and verify the migration steps before the actual production migration.
Manual method: Back up, copy, and then restore (without recovery) a full database backup, a differential backup (optional), and multiple transaction logs while keeping your production system operational. Continue taking and applying transaction log backups without recovery until you are ready to switch over. Apply the final transaction log backup with recovery when you are ready to switch your clients/applications over to connect to the SQL Server instance in the Azure virtual machine rather than the on-premises database.
AlwaysOn Availability Group method: If you have an on-premises AlwaysOn Availability Group, you can extend the availability group to SQL Server in your virtual machine. Configure your SQL Server instance in the Azure virtual machine as an AlwaysOn replica, seed the replica with a full database backup, keep the replica current with transaction log backups that are applied automatically, and then fail over to the Azure replica when you are ready to switch over your clients to the SQL Server instance in the Azure virtual machine (and turn off AlwaysOn in the source database).
Source of Information : Migrating SQL Server Databases to Azure