You will have to use DERIVED TABLE to do this

SELECTTOP 10 *

FROM(

SELECT key_value

,convert(datetime2,PV.key_value) as DT2

,convert(datetime,CONVERT(DATETIME2, PV.key_value)) AS DT

FROM parameter_value PV

WHERE PV.datatype = 'D'

) AS PV

WHEREconvert(datetime,CONVERT(DATETIME2, PV.key_value)) BETWEEN GETDATE() and DATEADD(d, 10, GETDATE())

This issue occurs because the query optimizer is evaluating the WHERE CLAUSE involving key_value first and then the one for datatype

Hence, its better to force it this way using a DERIVED TABLE.