There are 2 types of Report Viewer Web parts:
1. Web Parts which come with SQL Server Reporting Services
2. Web Parts which come with the Reporting Services Add-in for Sharepoint
Although both these sets of web parts have the same names, they are actually different. The difference between them is that the web parts which come with the add-in will work only with Reporting Services in SharePoint Integrated Mode.
The web parts which come with SQL Server Reporting Services work with Reporting Services in native mode.
The SSRS web parts can be found on the server where Reporting Services is installed. The default location is “C:\Program Files\Microsoft SQL Server\100\Tools\Reporting Services\SharePoint\RSWebParts.cab”. However these web parts are available in x86. Therefore, on a x64 server, the default location would be “C:\Program Files (x86)\Microsoft SQL Server\100\Tools\Reporting Services\SharePoint\ RSWebParts.cab”.
To install these web parts, copy the RSWebParts.cab file on the SharePoint Server and run the following command at the command prompt.
“STSADM.EXE -o addwppack -filename "RSWebParts.cab" -globalinstall”
Refer this link for more details: http://msdn.microsoft.com/en-us/library/ms159772.aspx
The Web Parts which come with the add-in are installed when the add-in is installed. Refer this link for more details: http://technet.microsoft.com/en-us/library/bb326356.aspx