grid view show multiple record at a time but in form view and detail view single row view is show.Basically grid view is replacement of data grid. In form view and detail view we can update,delete and insert record.but details view is often used in master-detail scenarios where the selected record in a master control such as a GridView control determines the record display.

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.gridview.aspxhttp://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.formview.aspx

The GridView control displays data as a table and provides the capability to sort columns, page through data, and edit or delete a single record. the GridView control offers improvements such as the ability to define multiple primary key fields, improved user interface customization using bound fields and templates, and a new model for handling or canceling events.

The FormView control renders a single record at a time from a data source and provides the capability to page through multiple records, as well as to insert, update, and delete records, similar to the DetailsView control. However, the difference between the FormView and the DetailsView controls is that the DetailsView control uses a table-based layout where each field of the data record is displayed as a row in the control. In contrast, the FormView control does not specify a pre-defined layout for displaying a record. Instead, you create templates that contain controls to display individual fields from the record. The template contains the formatting, controls, and binding expressions used to lay out the form.

Gridview shows the values of a  datasource in the form of a table whereas Formview shows the single record from the particular data source.

the only one difference between both is that grid view shows the data in grid format i.e.:- multiple records and form view shows only on one record .

everything else is similar both have same functionality

gridview wil be display multipal recorde at a time
but form and detailsview show recorde in single -sigle result set