We use Sitecore 9 Form. Is it possible to use xConnect to access databases (ExperienceForm Db) where are saved information from new Sitecore Forms module?
No it is not possible, xConnect is the service layer that sits in between the xDB and any trusted client, device, or interface that wants to read, write, or search xDB data. So you can not write to master,experience forms or other databases except xDB databases.
Communication is must happen over HTTPS and clients must have the appropriate certificate thumbprint.
More information about xConnect you can find at: https://doc.sitecore.net/developers/xp/xconnect/xconnect-overview/index.html
While you cannot use xConnect to access the Forms database, you can create custom save actions and save the data you want to retrieve onto the Contact in xDB and then use xConnect to get the data you saved.
The following documentation shows you how to configure Forms to update Contact details: