Use the csv module not lxml module to write rows to csv file. But still use lxml to parse and extract content from xml file:

import xml.etree.ElementTree as ET
import csv

tree = ET.parse('registerreads_EE.xml')

root = tree.getroot()[3]

with open('registerreads_EE.csv', 'w', newline='') as r:
    writer = csv.writer(r)
    writer.writerow(['read', 'date', 'entityid'])  # WRITING HEADERS

    for channel in tree.iter('Channel'):
        for exportrequest in channel.iter('ExportRequest'):
            entityid = exportrequest.attrib.get('EntityID')
            for meterread in channel.iter('Reading'):
                read = meterread.attrib.get('Value')
                date = meterread.attrib.get('ReadingTime')    

                # WRITE EACH ROW ITERATIVELY 
                writer.writerow([read[:-2],date[:10],entityid])  

Minor stuff:

• Remove the last import - etree is not used anywhere.
• Merge the two first imports

Possibly speed-improving stuff:

• Avoid converting the csv.reader output before returning unless absolutely necessary.
• Skip indent unless the output must be readable by a human with a non-formatting editor.
• If you need to indent the output, existing solutions are probably very efficient.
• Use reader.next() to skip the header line in generate_xml, then you don't need to keep checking the value of i.

The lxml library is capable of very powerful XML parsing, and can be used to iterate over an XML tree to search for specific elements.

from lxml import etree

with open(r'path/to/xml', 'r') as xml:
    text = xml.read()
tree = lxml.etree.fromstring(text)
row = ['', '']
for item in tree.iter('hw', 'def'):
    if item.tag == 'hw':
       row[0] = item.text
    elif item.tag == 'def':
       row[1] = item.text

line = ','.join(row)

with open(r'path/to/csv', 'a') as csv:
     csv.write(line + '\n')

How you build the CSV file is largely based upon preference, but I have provided a trivial example above. If there are multiple <dps-data> tags, you could extract those elements first (which can be done with the same tree.iter method shown above), and then apply the above logic to each of them.

EDIT: I should point out that this particular implementation reads the entire XML file into memory. If you are working with a single 150mb file at a time, this should not be a problem, but it's just something to be aware of.

You could create a list containing all the mappings you want. Try and search for each, and if it is not present, catch the AttributeError and store an empty value for it:

import xml.etree.ElementTree as ET
import csv

fields = [
    ('Id_Customer', 'Id_Customer'),
    ('Segment', 'Segment'),
    ('Nature', 'Event/Nature'),
    ('Extrainfo', 'Event/Extrainfo'),
    ('zipcode', 'Adress/zipcode'),
    ('street', 'Adress/street'),
    ('number', 'Adress/number')]

tree = ET.parse('cat.xml')
root = tree.getroot()

with open(r'customerdata.csv', 'wb') as f_customerdata:
    csv_customerdata = csv.DictWriter(f_customerdata, fieldnames=[field for field, match in fields])
    csv_customerdata.writeheader()

    for node in tree.iter('Customer'):
        row = {}

        for field_name, match in fields:
            try:
                row[field_name] = node.find(match).text
            except AttributeError as e:
                row[field_name] = ''

        csv_customerdata.writerow(row)

Giving you an output CSV file containing:

Id_Customer,Segment,Nature,Extrainfo,zipcode,street,number
xyz1,abc1,info1,info2,,,
zzwy,c2,,,77098,belaire drive,5

This approach also uses a DictWriter() instead of the standard csv writer. This makes it easier to assign values by name.

To cope with multiple address entries per customer, you first need to autocreate the maximum number of extra columns per entry. Then when accessing the elements, use findall() to get each one:

import xml.etree.ElementTree as ET
import csv

extra_columns = 2

fields = [
    ('Id_Customer', 'Id_Customer', 1),
    ('Segment', 'Segment', 1),
    ('Nature', 'Event/Nature', 1),
    ('Extrainfo', 'Event/Extrainfo', 1),
    ('zipcode', 'Adress/zipcode', extra_columns),
    ('street', 'Adress/street', extra_columns),
    ('number', 'Adress/number', extra_columns)]

tree = ET.parse('cat.xml')
root = tree.getroot()

# Auto create the header from fields
fieldnames = []

for field, match, cols in fields:
    fieldnames.append(field)

    if cols > 1:
        fieldnames.extend(["{}{}".format(field, x+2) for x in range(extra_columns)])

with open(r'customerdata.csv', 'wb') as f_customerdata:
    csv_customerdata = csv.DictWriter(f_customerdata, fieldnames=fieldnames)
    csv_customerdata.writeheader()

    for node in tree.iter('Customer'):
        row = {}

        for field_name, match, cols in fields:
            if cols > 1:
                for index, el in enumerate(node.findall(match)):
                    try:
                        if index:
                            row["{}{}".format(field_name, index+1)] = el.text
                        else:
                            row[field_name] = el.text

                    except AttributeError as e:
                        row[field_name] = ''
            else:
                try:
                    row[field_name] = node.find(match).text
                except AttributeError as e:
                    row[field_name] = ''

        csv_customerdata.writerow(row)

So your header would now look like:

Id_Customer,Segment,Nature,Extrainfo,zipcode,zipcode2,zipcode3,street,street2,street3,number,number2,number3

My problem is that I don't know how to access CDATA content. Because TEXT in some DOCs has an IMAGE child

The below seems to work. The code handle the cases of IMAGE under TEXT and TEXT with no IMAGE under it.

import xml.etree.ElementTree as ET

xml = '''<?xml version="1.0" encoding="UTF-8"?>
<root>
   <DOC>
      <TEXT>
         <IMAGE>/1379/791012/p18-1.jpg</IMAGE>
         <![CDATA[The section I want to access to]]>
      </TEXT>
      <TEXT>
         <![CDATA[more text]]>
      </TEXT>
   </DOC></root>'''

root = ET.fromstring(xml)
texts = root.findall('.//TEXT')
for idx, text in enumerate(texts, start=1):
    data = list(text)[0].tail.strip() if list(text) else text.text.strip()
    print(f'{idx}) {data}')

output

1) The section I want to access to
2) more text