The value space of xs:float is “float” (32 bits) floating-point numbers as defined by the IEEE. The lexical space uses a decimal format with optional scientific notation. The match between lexical (powers of 10) and value (powers of 2) spaces is approximate and is done on the closest value.

This datatype differentiates positive (0) and negative (-0) zeros, and includes the special values “-INF” (negative infinity), “INF” (positive infinity), and “NaN” (Not a Number).

Note that the lexical spaces of xs:float and xs:double are exactly the same; the only difference is the precision used to convert the values in the value space. The decimal separator is always a point (“.”) and no thousands separator may be added

Valid values include: "123.456", "+1234.456", "-1.2344e56", "-.45E-6", "INF", "-INF", or "NaN".
The following values would be invalid: "1234.4E 56" (spaces are forbidden), "1E+2.5" (the power of 10 must be an integer), "+INF" (positive infinity doesn’t expect a sign), or "NAN" (capitalization matters in special values).

The formal definition of this datatype is at: http://www.w3.org/TR/xmlschema-2/#float